Error when trying to join a group i was invited to

I got invited to a private group and I tried to confirm but it wont let me. An error message pops up saying "an error occured while trying to join this group". Its a group with all my highschool classmates which would be nice to be able to join. Can anyone help me as to why this isn't working? and is there a way to fix the problem?

Try clearing your cookies and cache.

Try with iFacebook said, but also try logging in and out. It should work then.