In Arma differents groups can't communicate directly, just ask support with a team or many with scripted mission. It's a lack, when differents teams have the same objective that they can't sync movement, attack or communicate on danger ( fire on a team ). Groups leaders have to work together to be more efficients.
Now when you lost your team medic or you are the last survivor of your team, you're alone, impossible to call a medic from another team.
Add in communication system a key ( like a combination key with Ctrl, shift ) to communicate with anothers groups, and possibility to groups leaders to communicate together, like the side channel in MP.
Another thing, add differents ranks to groups leaders, like in real life, to let the highest rank group leader lead an attack to sync it.