Bring back join/leave button Ajax feature in Groups Directory

After r8378 the use of '.' instead of ',' at line 1244 of buddypress.js file was causing the ajax feature to fail.

fixes #5624

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8399 cdf35c40-ae34-48e0-9cc9-0c9da1808c22