I'm trying to make a request to the table Group_Members.
I get an error "[User Error] Bad class to singleton() - Group_Members"

Line 261 in /var/www/***/subdomains/test2/httpdocs/sapphire/core/Core.php
Source

252 *
253 * @param string $className
254 * @return Object
255 */
256 function singleton($className) {
257 global $_SINGLETONS;
258 if(!isset($className)) user_error("singleton() Called without a class", E_USER_ERROR);
259 if(!is_string($className)) user_error("singleton() passed bad class_name: " . var_export($className,true), E_USER_ERROR);
260 if(!isset($_SINGLETONS[$className])) {
261 if(!class_exists($className)) user_error("Bad class to singleton() - $className", E_USER_ERROR);
262 $_SINGLETONS[$className] = Object::strong_create($className,null, true);
263 if(!$_SINGLETONS[$className]) user_error("singleton() Unknown class '$className'", E_USER_ERROR);
264 }
265 return $_SINGLETONS[$className];
266 }

There is no ClassName field in the table. How to make a request?

Because Group_Members isn't a dataobject. That table is a relationship table made up of groups -> member relationships.

If you want to get all the groups which have a member you can call Groups() on a member object, and if you want all the members in a group you can call Members() on a group object.