In Sakai 2.7 the membership tool seems to ignore these sakai properties that were working in Sakai 2.6:
I believe this is related to:
It looks like the problem is in this file: /site-manage/site-manage-tool/tool/src/webapp/vm/membership/chef_membership.vm
The check to see if you can unjoin a site uses the $siteType variable which is declared below the if statement that uses it. I moved the variable declaration back above the if statement where it used to be and everything works again. Since I'm not good at explaining things I have attached the diff of the code that is working for me.