Restrict Joining More Than One Group

yz_restrict_group_join.php

<?php
/**
 * Restrict Joining More Than One Group
 */
function yz_restrict_group_join( $button, $group ) {
	global $bp;
	if ( current_user_can( 'manage_options' ) || $group->is_member ) {
		return $button;
	}
	
	$membership_total = count( BP_Groups_Member::get_membership_ids_for_user( $bp->loggedin_user->id ) );
	
	if ( $membership_total >= 1 ) {
		return false;
	}
	
	return $button;
}
add_filter( 'bp_get_group_join_button', 'yz_restrict_group_join', 10, 2 );