If you want 2 people to join the group at the same time you can just use:
And to make it work properly in MP you probably want to also name the player and do:
[man, man2] join player
That is, assuming man and man2 are AI and their group is lead by an AI (which is the case if they are in a 1-man group, obviously) so that they are local to the server. Note that player is null on a dedicated server and will be a different unit every time on a hosted server (as the host could play the unit you want them to join or he could play something else that you don't want them to join).
if (isServer) then
[man, man2] join nameofplayer;