Index: roster-app/src/java/org/sakaiproject/tool/roster/FilteredParticipantListingBean.java
===================================================================
--- roster-app/src/java/org/sakaiproject/tool/roster/FilteredParticipantListingBean.java	(revision 329)
+++ roster-app/src/java/org/sakaiproject/tool/roster/FilteredParticipantListingBean.java	(working copy)
@@ -23,6 +23,7 @@
 import java.io.Serializable;
 import java.text.MessageFormat;
 import java.util.ArrayList;
+import java.util.Collection;
 import java.util.Iterator;
 import java.util.List;
 import java.util.SortedMap;
@@ -124,8 +125,9 @@
 		return getSearchFilterString() != null && ! getSearchFilterString().equals(defaultSearchText) && ! searchMatches(getSearchFilterString(), participant.getUser());
 	}
 
-	protected SortedMap<String, Integer> findRoleCounts(List<Participant> participants) {
+	protected SortedMap<String, Integer> findRoleCounts(Iterable<Participant> participants) {
 		SortedMap<String, Integer> roleCountMap = new TreeMap<String, Integer>();
+		
 		for(Iterator<Participant> iter = participants.iterator(); iter.hasNext();) {
 			Participant participant = iter.next();
 			String role = participant.getRoleTitle();
Index: roster-app/src/java/org/sakaiproject/tool/roster/RosterGroupMembership.java
===================================================================
--- roster-app/src/java/org/sakaiproject/tool/roster/RosterGroupMembership.java	(revision 329)
+++ roster-app/src/java/org/sakaiproject/tool/roster/RosterGroupMembership.java	(working copy)
@@ -64,48 +64,61 @@
 		return filter.isGroupedBy();
 	}
 	
-	public List<GroupedParticipants> getGroupedParticipants() {
-        Site site;
+	@SuppressWarnings("unchecked")
+	public Collection<GroupedParticipants> getGroupedParticipants() {
+		List<GroupedParticipants> groupedParticipants = null;
+        Site site = null;
+
 		try {
 			site = filter.services.siteService.getSite(filter.getSiteReference().substring(6));
 		} catch (IdUnusedException e) {
 			log.error("Unable to find site for: " + getSiteReference() + " " + e.getMessage(), e);
 			return null;
 		}
-        List<GroupedParticipants> groupedParticipants = new ArrayList<GroupedParticipants>();
-        Collection groups = site.getGroups();
-        for(Iterator<Group> groupIter = groups.iterator(); groupIter.hasNext();)
-        {
-            Group group = groupIter.next();
-            List<Participant> roster = filter.services.rosterManager.getRoster(group.getReference());
-            groupedParticipants.add(new GroupedParticipants(group.getTitle(), roster, roster.size(), getRoleCountMessage(filter.findRoleCounts(roster))));
-        }
-        
-        List<Participant> fullRoster = filter.services.rosterManager.getRoster();
-        List<Participant> unassignedList = new ArrayList();
-        // checking if there's any participants who are not in any groups
-        for(Iterator<Participant> partIter = fullRoster.iterator(); partIter.hasNext();)
-        {
-        	Participant participant = partIter.next();
-        	boolean userGrouped = filter.services.rosterManager.isParticipantGrouped(participant.getUser().getId());
-        	if (!userGrouped)
-        	{
-        		unassignedList.add(participant);
-        	}
-        }
-        // if we have participants who are ungrouped, we add them here to a new one for rendering called "Unassigned"
-        if (!unassignedList.isEmpty())
-        {
-        	String unassigned = "Unassigned";
-        	groupedParticipants.add(new GroupedParticipants(unassigned, unassignedList, unassignedList.size(), getRoleCountMessage(filter.findRoleCounts(unassignedList))));
-        }
-
-        Collections.sort(groupedParticipants, sortByGroup());
+		
+		if (site != null) {
+	        groupedParticipants = new ArrayList<GroupedParticipants>();
+	        Collection<Group> groups = (Collection<Group>) site.getGroups();
+	        
+	        //Use a HashSet because we'll have to use .removeAll() on it many times
+	        //.remove() is roughly constant time for a HashSet.
+	        Set<Participant> unassignedParticipants = new HashSet<Participant>(
+	        		filter.services.rosterManager.getRoster()
+	        	);
+	        
+	        for(Iterator<Group> groupIter = groups.iterator(); groupIter.hasNext();)
+	        {
+	            Group group = groupIter.next();
+	            List<Participant> roster = filter.services.rosterManager.getRoster(group.getReference());
+	            
+	            //remove each grouped participant from the 'unassignedParticipants' set
+	            unassignedParticipants.removeAll(roster);
+	            
+	            groupedParticipants.add(new GroupedParticipants(group.getTitle(), roster, roster.size(), getRoleCountMessage(filter.findRoleCounts(roster))));
+	        }
+	        
+	        // if we have participants who are ungrouped, we add them here to a new one for rendering called "Unassigned"
+	        if (!unassignedParticipants.isEmpty())
+	        {
+	        	String unassigned = "Unassigned";
+	        	groupedParticipants.add(
+	        			new GroupedParticipants(
+	        					unassigned, 
+	        					unassignedParticipants, 
+	        					unassignedParticipants.size(), 
+	        					getRoleCountMessage(filter.findRoleCounts(unassignedParticipants))
+	        				)
+	        		);
+	        }
+			
+	        Collections.sort(groupedParticipants, sortByGroup());
+		}
+		
         return groupedParticipants;
 	}
 	
 	public class GroupedParticipants {
-        List<Participant> groupedParticipants = new ArrayList<Participant>();
+		Collection<Participant> groupedParticipants = new ArrayList<Participant>();
         String groupTitle;
         int groupedParticipantCount;
         String roleCountMessage;
@@ -122,10 +135,10 @@
 		public void setGroupTitle(String groupTitle) {
 			this.groupTitle = groupTitle;
 		}
-		public List<Participant> getGroupedParticipants() {
+		public Collection<Participant> getGroupedParticipants() {
 			return groupedParticipants;
 		}
-		public void setGroupedParticipants(List<Participant> groupedParticipants) {
+		public void setGroupedParticipants(Collection<Participant> groupedParticipants) {
 			this.groupedParticipants = groupedParticipants;
 		}
 		public String getRoleCountMessage() {
@@ -137,7 +150,7 @@
 		
 		public GroupedParticipants() {}
         
-		public GroupedParticipants(String groupTitle, List<Participant> groupedParticipants, int groupedParticipantCount, String roleCountMessage) {
+		public GroupedParticipants(String groupTitle, Collection<Participant> groupedParticipants, int groupedParticipantCount, String roleCountMessage) {
 			this.groupTitle = groupTitle;
 			this.groupedParticipants = groupedParticipants;
 			this.groupedParticipantCount = groupedParticipantCount;
Index: roster-impl/src/java/org/sakaiproject/component/app/roster/ParticipantImpl.java
===================================================================
--- roster-impl/src/java/org/sakaiproject/component/app/roster/ParticipantImpl.java	(revision 329)
+++ roster-impl/src/java/org/sakaiproject/component/app/roster/ParticipantImpl.java	(working copy)
@@ -130,4 +130,31 @@
 		}
 		return groupsString;
 	}
+	
+	/* (non-Javadoc)
+	 * @see java.lang.Object#hashCode()
+	 */
+	@Override
+	public int hashCode() {
+		return user.hashCode();
+	}
+	
+	/* (non-Javadoc)
+	 * @see java.lang.Object#equals(java.lang.Object)
+	 */
+	@Override
+	public boolean equals(Object obj) {
+		boolean rv = false;
+		Participant p = null;
+		
+		if (this == obj) {
+			rv = true;
+		} else if (user != null && obj != null && obj instanceof Participant) {
+			p = (Participant) obj;
+		
+			rv = user.equals(p.getUser());
+		}
+		
+		return rv;
+	}
 }