It's a doubles event so you'll need to buddy up with someone who hasn't turned on GTA's campaign just yet. It's running from like, 9PM to 12AM here in the UK (from 4PM EST).
You'll need to add the following PSN names:
Then obviously access the multiplayer via Niko's phone and hit Team Deathmatch.
We won't be playing though. We like Grand Theft Auto IV plenty (we ain't no hater, yo) but the multiplayer is utter tripe.