Extends the [groups_user_groups] shortcode to display the ids. This new shortcode is [groups_user_groups_id]

functions.php

add_shortcode( 'groups_user_groups_id', 'itx_groups_user_groups_id' );
/**
 * Renders the current or a specific user's groups id.
 * Attributes:
 * - "user_id" OR "user_login" OR "user_email" to identify the user, if none given assumes the current user
 * - "format" : one of "list" "div" "ul" or "ol" - "list" and "ul" are equivalent
 * - "list_class" : defaults to "groups"
 * - "item_class" : defaults to "name"
 * - "order_by"   : defaults to "name", also accepts "group_id"
 * - "order"      : default to "ASC", also accepts "asc", "desc" and "DESC"
 *
 * @param array $atts attributes
 * @param string $content not used
 * @return rendered groups for current user
 */
function itx_groups_user_groups_id( $atts, $content = null ) {
		$output = '';
		$options = shortcode_atts(
			array(
				'user_id' => null,
				'user_login' => null,
				'user_email' => null,
				'format' => 'list',
				'list_class' => 'groups',
				'item_class' => 'name',
				'order_by' => 'name',
				'order' => 'ASC',
				'group' => null,
				'exclude_group' => null
			),
			$atts
		);
		$user_id = null;
		if ( $options['user_id'] !== null ) {
			if ( $user = get_user_by( 'id', $options['user_id'] ) ) {
				$user_id = $user->ID;
			}
		} else if ( $options['user_id'] !== null ) {
			if ( $user = get_user_by( 'login', $options['user_login'] ) ) {
				$user_id = $user->ID;
			}
		} else if ( $options['user_email'] !== null ) {
			if ( $user = get_user_by( 'email', $options['user_login'] ) ) {
				$user_id = $user->ID;
			}
		}
		if ( $user_id === null ) {
			$user_id = get_current_user_id();
		}
		if ( $user_id !== null ) {
			$user = new Groups_User( $user_id );
			$groups = $user->groups;

			if ( !empty( $groups ) ) {
			// group attr
				if ( $options['group'] !== null ) {
					$groups = array();
					$groups_incl = explode( ',', $options['group'] );
					foreach ( $groups_incl as $group_incl ) {
						$group = trim( $group_incl );
						$current_group = Groups_Group::read( $group );
						if ( !$current_group ) {
							$current_group = Groups_Group::read_by_name( $group );
						}
						if ( $current_group ) {
							if ( Groups_User_Group::read( $user_id, $current_group->group_id ) ) {
								$groups[] = $current_group;
							}
						}
					}
				}
				// exclude_group attr
				if ( $options['exclude_group'] !== null ) {
					$groups_excl = explode( ',', $options['exclude_group'] );
					foreach ( $groups_excl as $key => $group_excl ) {
						$group = trim( $group_excl );
						$current_group = Groups_Group::read( $group );
						if ( !$current_group ) {
							$current_group = Groups_Group::read_by_name( $group );
						}
						if ( $current_group ) {
							$groups_excl[$key] = $current_group->group_id;
						} else {
							unset( $groups_excl[$key] );
						}
					}
					foreach ( $groups as $key => $group ) {
						if ( in_array( $group->group_id, $groups_excl ) ) {
							unset( $groups[$key] );
						}
					}
				}
				switch( $options['order_by'] ) {
					case 'group_id' :
						usort( $groups, array( __CLASS__, 'sort_id' ) );
						break;
					default :
						usort( $groups, array( __CLASS__, 'sort_name' ) );
				}
				switch( $options['order'] ) {
					case 'desc' :
					case 'DESC' :
						$groups = array_reverse( $groups );
						break;
				}

				switch( $options['format'] ) {
					case 'list' :
					case 'ul' :
						$output .= '<ul class="' . esc_attr( $options['list_class'] ) . '">';
						break;
					case 'ol' :
						$output .= '<ol class="' . esc_attr( $options['list_class'] ) . '">';
						break;
					default :
						$output .= '<div class="' . esc_attr( $options['list_class'] ) . '">';
				}
				foreach( $groups as $group ) {
					switch( $options['format'] ) {
						case 'list' :
						case 'ul' :
						case 'ol' :
							$output .= '<li class="' . esc_attr( $options['item_class'] ) . '">' . $group->name . ' (' . $group->group_id . ')' . '</li>';
							break;
						default :
							$output .= '<div class="' . esc_attr( $options['item_class'] ) . '">' . $group->name . ' (' . $group->group_id . ')' . '</div>';
					}
				}
				switch( $options['format'] ) {
					case 'list' :
					case 'ul' :
						$output .= '</ul>';
						break;
					case 'ol' :
						$output .= '</ol>';
						break;
					default :
						$output .= '</div>';
				}
			}
		}
		return $output;
}