groups_accept_membership_request( int $membership_id, int $user_id, int $group_id )

function groups_accept_membership_request( $membership_id, $user_id = 0, $group_id = 0 ) {

	if ( !empty( $user_id ) && !empty( $group_id ) ) {
		$membership = new BP_Groups_Member( $user_id, $group_id );
	} else {
		$membership = new BP_Groups_Member( false, false, $membership_id );
	}

	$membership->accept_request();

	if ( !$membership->save() ) {
		return false;
	}

	// Check if the user has an outstanding invite, if so delete it.
	if ( groups_check_user_has_invite( $membership->user_id, $membership->group_id ) ) {
		groups_delete_invite( $membership->user_id, $membership->group_id );
	}

	/**
	 * Fires after a group membership request has been accepted.
	 *
	 * @since BuddyPress 1.0.0
	 *
	 * @param int  $user_id  ID of the user who accepted membership.
	 * @param int  $group_id ID of the group that was accepted membership to.
	 * @param bool $value    If membership was accepted.
	 */
	do_action( 'groups_membership_accepted', $membership->user_id, $membership->group_id, true );

	return true;
}