I don't think that it would be really useful to have some sort of chat on the forums. Just use the PM system, it's there for a reason. If you want a live chat that you would join with your forum buddies and it's GTA related then you can use IRC. Go there
, then choose "GTANET [webirc]" to connect with the GTANet server, then type in the Nickname you want to use, and then choose a channel. You have to type in the name of the channel by yourself so if you want to talk about GTA V then you have to write "#gta5" (without quotes) and of course, you can choose any channel you want provided you know what the name of the servers, you want to join, exactly is. You can search for the channels by yourself and you my use this topic
for more informations.