bp_group_is_member( object|bool $group = false )

function bp_group_is_member( $group = false ) {
	global $groups_template;

	// Site admins always have access.
	if ( bp_current_user_can( 'bp_moderate' ) ) {
		return true;
	}

	if ( empty( $group ) ) {
		$group =& $groups_template->group;
	}

	/**
	 * Filters whether current user is member of a group.
	 *
	 * @since BuddyPress 1.2.4
	 * @since BuddyPress 2.5.0 Added the `$group` parameter.
	 *
	 * @param bool   $is_member If user is a member of group or not.
	 * @param object $group     Group object.
	 */
	return apply_filters( 'bp_group_is_member', ! empty( $group->is_member ), $group );
}