There is no full-fledged midgame join at the moment. You can use cheats to emulate it but the new guy will have to share units with an existing player (like in SC2 archon mode) and it requires manual intervention.
First, decide whom the new guy will join. You can say
/info
which toggles a list of people's IDs. Enable cheats
!cheats
Then the new guy just does
/team <id>
with cheats enabled.