From my understanding an existing infantry team can dismount the machine gun - it is not an extra team. Only the .50 cals acts as a gun teams, the LMG act as regular US LMG team (move like infantry - can't move and shoot, 3+ save, 2 shots when pinned).
The only way to have these units return to their old self is to remount the half track and then dismount without the weapon.
I do not have experience with it, but I have heard that this is devastating in defense when the US basically starts all unmounted and dug in with half tracks in the rear. An armored infantry platoon ends up with like 4-5 machine guns, and a 37mm gun, dug in.
Not many things are going to want to close in on that.