Improve logic for adding a user to a group in the Dashboard

On submit, the JS converts the dynamically created <li> items to a proper
comma-separated list of user logins. This creates greater consistency with the
noscript workflow, and also fixes a bug where users who were found via
autocomplete but then removed from the list of users to add would be added
anyway.

Fixes #5633

Props imath

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8417 cdf35c40-ae34-48e0-9cc9-0c9da1808c22