You won't be able to do that using CBSubs GroupJive. You'll probably need to use CB Auto Actions for that and act on the CBSubs Family Plans triggers, which are as follows.
Code:
$_PLUGINS->trigger( 'familyplans_onAfterAcceptSharedSub', array( $this, $user, $subscription ) );
$_PLUGINS->trigger( 'familyplans_onAfterDeleteSharedSub', array( $this, $user, $subscription ) );
$_PLUGINS->trigger( 'familyplans_onCanAcceptSharedSub', array( &$this, $userId, $subscription ) );
$_PLUGINS->trigger( 'familyplans_onBeforeAcceptSharedSub', array( &$this, $user, $subscription ) );
familyplans_onAfterAcceptSharedSub would be what you're use to have them join the group. You'll probably have to use the New Group mode though and set Unique to No and Auto Join to Yes in the CB GroupJive auto action.