I don't see any options in my game to join or invite. I did read one of the patch notes about inviting party members but I need some help on this.
There is co op, you can either type /invite Name or there is a friends list in game I beleive if you check out the bottom of the screen, you can also send invites through steam I think. Currently 4 player co op is enabled.
You can invite someone to your party by clicking their name in chat.