Hi there,
if a multiplayer game is running you can join as player or spectator.
In both cases you will be a spectator.
How about joining as player will join you to the running game with a commander.
Or if there is a disconnect/left Player so you can take his Units like reconnecting.
Of Course this should be allowed by the ingame Player like a vote with give Units of xxx to yyy or join with new own commander.
Regards,
Elgaron