Add users to Organic Groups using correct function call

When users are added and removed to organic groups via the civicrm_og_sync module, they are added using a called to og_membership_create(). This adds them as a member, but subsequently makes it impossible to remove them from the group via the OG interface.

A call to og_group() and og_ungroup() for adding a removing users respectively fixes this issue, and is a better function to use in this case.