If there is an event on your platform, you just give a reply to the tread on this forum saying something like: "I'll be there!" or "Sign me up". Then you usually send the host of the event a friend request so you can join his GTA session. The host opens a pre-lobby for the event approximately 30 min before the start of the event. And 5 min before the events starts, he starts the race and invites everyone that was in the pre-lobby. I hope this is enough information to help you join events!