Iirc you need to:
1) On the computer running your save, press escape
2) click Manage Players
3) Open the slot you want to be able to join
4) Click Apply
5) On the computer you want to join from, make sure you're using a profile with the same name as the player name in the open slot (if the slot hasn't had a player in it previously and just shows
, then your profile name can be whatever)
6) Join the game
At least, I think that will let you do what you're asking
If you're trying to transfer host then you'll need to save, stop the game, and restart it on the machine you want to host.