How to add user to OG 7.x-2.x group with rules?

Depending upon the type of registration you offer - (without verification, with verification, admin setup etc) rules will not catch all scenarios (my experience anyway).

The only way I have found to automate something like this is to include checks like this in my own custom module where I have other checks and tweeks. I have a predefined log in redirect page that fires off these checks. On this page I have a block that calls scripts to ensure that the user is up to date on what I want them to be. Yes this is overhead, however this gives me automated access to new and existing accounts as well.

Untested query but should work (GID and UID already set):
$og_mem = db_query("SELECT * FROM og_membership WHERE etid = :etid AND gid = :gid", array('etid'=>$uid, 'gid'=>$gid));
$number_of_rows = $og_mem->rowCount();

if($number_of_rows == 0){
$into_group = db_insert('og_membership')
->fields(array('type', 'etid', 'entity_type', 'gid', 'group_type', 'state', 'created', 'field_name', 'language'))
->values(array(
'type' => 'og_membership_type_default',
'etid' => $uid,
'entity_type' => 'user',
'gid' => $gid,
'group_type' => 'node',
'state' => '1',
'created' => REQUEST_TIME,
'field_name' => 'og_user_node',
'language' => 'en',
))->execute();
}