Like most WWII-based games, DoD encourages players to operate as part of a squad and it can be very enjoyable to support your friends, with the added benefit that it's a lot easier to do well if you're being supported by your team. Below are a few points that will help you make the most of your teammates (and yourself).
Obviously, you're not going to do well as part of a unit if you're nowhere near your squad. Depending on the map, you should try and bunch up in an appropriate size of squad with a decent mix of light and heavy infantry; the heavies should lead the line with the riflemen providing supporting fire. Snipers and gunners should take up helpful positions where they can hold back enemy advances or help the squad by taking out difficult-to-hit enemies. If you're assisting rocket troops, try and clear a path for them as they are very vulnerable on the move.
Camping can be a big temptation to many people, with a lower chance of getting killed and a higher chance of picking enemies off unawares. However, is covering that remote doorway on the flanks really helping your team? And do you need the sole sniper rifle to do it? If you are going to camp, try and do it somewhere where you can help your squadmates out - this is good for you too as it diverts attention from your position.
Communication is one of the most important ways you can help your team. Just been picked off by a sniper? Let your team know where he is so they don't fall into the same trap. Just spotted an enemy sneaking up on an objective? Shout out his position so everyone around you knows where to look. Need a hand capping an objective? Ask your teammates to help - they might not know you need backup. Much of this can be achieved using the voice menus or plain typed chat messages, but it's far easier if you either use a microphone to talk to your squad in realtime (here it's best to remember to be polite - positive mental attitude and all that), or better still use some binds to let your team know what's going on.