groups_accept_invite( int $user_id, int $group_id )

function groups_accept_invite( $user_id, $group_id ) {

	// If the user is already a member (because BP at one point allowed two invitations to
	// slip through), delete all existing invitations/requests and return true.
	if ( groups_is_user_member( $user_id, $group_id ) ) {
		if ( groups_check_user_has_invite( $user_id, $group_id ) ) {
			groups_delete_invite( $user_id, $group_id );
		}

		if ( groups_check_for_membership_request( $user_id, $group_id ) ) {
			groups_delete_membership_request( null, $user_id, $group_id );
		}

		return true;
	}

	$member = new BP_Groups_Member( $user_id, $group_id );

	// Save the inviter ID so that we can pass it to the action below.
	$inviter_id = $member->inviter_id;

	$member->accept_invite();

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

	// Remove request to join.
	if ( $member->check_for_membership_request( $user_id, $group_id ) ) {
		$member->delete_request( $user_id, $group_id );
	}

	// Modify group meta.
	groups_update_groupmeta( $group_id, 'last_activity', bp_core_current_time() );

	/**
	 * Fires after a user has accepted a group invite.
	 *
	 * @since BuddyPress 1.0.0
	 * @since BuddyPress 2.8.0 The $inviter_id arg was added.
	 *
	 * @param int $user_id    ID of the user who accepted the group invite.
	 * @param int $group_id   ID of the group being accepted to.
	 * @param int $inviter_id ID of the user who invited this user to the group.
	 */
	do_action( 'groups_accept_invite', $user_id, $group_id, $inviter_id );

	return true;
}