1. As site maintainer, create a group (Site Info -> Manage Groups -> New)
2. Select Matrices and select a matrix
The Manage Matrices drop-down menu will not appear
3. Return to Site Info -> Manage Groups and remove the group that was added in step 1 above
4. Return to Matrices and select the matrix.
The Manage Matrices drop-down now appears.
I haven't done much with groups, but I'm pretty sure this is new behavior.