Use ElggUser entity instead of guid to check group membership in add action.

If we pass in $who->guid then ElggGroup::isMember() ignores it and instead checks to see if the logged-in user is a member of the group.  I think the intent is to check whether $who is a member of the group.  This bug prevents the site administrator from being able to grant group users admin privileges in their groups (unless the administrator happens to be a member of the group in question).