Its in 4 hours time, I don't think you need to be part of the social club to take part.
This is a paste job from the site:
HOW TO FIND US
Grab a multiplayer partner and start up Team Deathmatch via Party Mode using the in-game cellphone. We're playing doubles so make sure your team size is two. By starting up Team Deathmatch from Party Mode, you will be automatically placed in special lobbies together with us and other event participants. More info on using Party Mode is listed below.
NEED A PARTNER?
Hit up the chatbox during the event or just post your Gamertag in the Comments section below. Someone will pick ya up.
ABOUT PARTY MODE
Party Mode is an easy way to get your group together and find matches. Here are the 3 quick and easy steps to get you up and running:
Save your current game. To get started, fire up the in-game mobile phone by pressing "Up" on the keyboard. Select Multiplayer > Ranked Match > Quick Match > Party Mode > Yes.
From here, open up your phone again and select the Friends option to invite your friends (you need at least one more friend to play with Rockstar).
Once your team is ready to roll, open up the in-game phone again and select
Games > Team Deathmatch for this event.
Again, please be sure that you enter the lobby in a group of two if you plan to get matched up against us and other players for the event.