Communication, Communication, Communication...
It doesn't matter how you do it, just do it...
Hand signals are great if your trying to be stealthy, but make sure everyone knows what the hell your trying to say with your hands... And once your cover has been blown, there is no point in trying to keep quiet... This doesn't mean to start trying to have a conversation in the middle of a firefight... It means, start yelling out positions for your teammates to start looking for... Don't give away your teammates positions, but make sure they know what your trying to tell them...
If you have radios, this becomes a lot easier... But keep in mind that radios can also give you away... Unless you have the high dollar CB radios (and the license to use them) then your radio can be listened in on... Once again, when you get into the heat of the battle, get off the radio and start yelling out enemy positions...
And Please... Please.... Don't be a CHATTERBOX on the radio, you might give your friend/teammates position away when he's surrounded by X-number of enemies... Plus it can also keep you from getting important intel when the comm's are cluttered up by people having conversations instead of what they're supposed to be doing...