You'll have to actually be connected to a server - in the mission - and have picked a side for you to tell.
...when you do, you'll see a green light on radio (and amber on intercom), you'll see inside the bar, the channel you're on and the number of folks in that channel ( example Channel 1 - 3 = Channel 1 - 3 people on the channel)
Then when someone talks, you'll see their name in the bar. Voila'.... you're all set.
Played around with Tank Crew and DetCord12B's "Achtung Panzer!" MOD. I think @WWICYtheDEAD might like this shot...
Original:
Final Edit:
Comic book style: