Add endpoint to get current users in a group

Added an endpoint for /accounts/group/get_group_users to be able to pull the user IDs of current members of a given group ID. 

/accounts/group/get only returns some meta info about the group itself, not its members. 

Hitting /accounts/group_user/add and /accounts/group_user/delete allows you to add/remove users from an group, but there wasn't a way to grab the current membership. Built off of the existing VBX_Group::get_user_ids method.