I think there should be a button(inbetween sign in and no tags set) saying "multilayer mode". When clicked it will open a Window listing your groups and avaliable players. If you click on a (online) player it will send a request, if the player accepts it another pointer will appear on the screen with the player name above it. Now you can build things together and work on prodgects. If you click on a group the same thing happens only the request is sent to all group members ect. To tell each other what to do press the / key and type a message(press enter to send) and what you typed will appear under the pointer for 30 seconds