On May 22nd, Rockstar released a patch to fix a number of known-issues in Multiplayer mode. One of the issues supposedly resolved was (1) inability to join Multiplayer; and, (2) sudden disconnection from Multiplayer.
I don't believe this issue has been fully resolved.
Since purchasing Read Dead Redemption I have successfully logged into Multiplayer TWICE. All other attempts have netted me an error message. After confirming the message, I get returned to the Multiplayer loading screen, which never loads. I must restart the game.
I have attempted, once or twice per day, to gain access to Multiplayer and always the same error occurs.
I have a 6Mbps connection download speed, so I doubt the problem is related to network connectivity.
I am playing on an XBox with a recently-confirmed 12 Month Gold Membership.