Alot of forgers do planned Lobbies on HaloCustoms.com. I know Squally and Duck run weekly testing Lobbies and there are other forgers who have their own ways. From there just network with forgers from the Lobbies so you'll be able to have a bunch of impromptu lobbies
In addition to what's already been posted, there are weekly testing groups on Forgehub that you can read about here - http://www.forgehub.com/threads/alliance-testing-network-info-contact-thread.151787/