You should assign a role to each member of your team. If your team has more than 5 members, then dividing everyone into squads of 5 or 6 men each is a good idea. Here are the roles:
Assault soldier (rifleman): The assault soldier forms the backbone of the squad. He fights with a medium range assault rifle. His main responsibility is to act as the squad's muscle, eliminating hostile forces and helping the team achieve its objective through flanking, suppressive fire, and tactical movement. They should move swiftly and have a quick mind to evaluate threats and respond accordingly. They should also take orders willingly and never flee or retreat unless commanded to do so by the squad leader.
Point man: If the squad is a spear, then the point man is the very tip of it. Armed with a rifle or submachine gun, he goes ahead of the assault soldiers, acting as recon and team leader. He must be the squad's best tactician, in order to decide when to and when not to engage. Stealthy movement is also a must, along with leadership skills to keep the squad in line.
Support Gunner: The support gunner has one simple job: suppress the enemy so that the assault team can move unmolested. In order to complete this, he must have a large, powerful automatic weapon. His aim and tactical sense need not be great, as his role is only to put a huge amount of lead into the right area. However, he should be strong in order to carry his heavy loadout of heavy guns and magazines.
Designated Marksman: The designated marksman is a part of the assault team. However, he is different then his fellow assault grunts because of his weapon, a large, accurate semiautomatic or automatic rifle. He has two roles. The first is to engage targets that are out of the assault team's range, hurting the enemy before they can do any damage. His second role is as a countersniper, to find and suppress enemy snipers as the squad moves in and destroys them.
Sniper/spotter: As the most independent soldier in the squad, the sniper is largely autonomous from the assault team. With his large, accurate rifle, his main job is to harrass and harry the enemy by attacking them from an unknown position. As a secondary duty, the sniper reports back to the squad, via radio or hand signals, about enemy movements andpositions. He should be the stealthiest person on the squad, skilled in woodcraft and concealment. The sniper should also have a slightly "lone wolf" mentality, as he occasionally will be completely cut off from the squad and must survive alone.
Heavy Weapons/Demolition Man: This soldier's main duty is intimidation, suppressing the enemy with a barrage of explosives and smoke to cover up the squad's movements. It is definitely not a position that requires finesse.
A squad should be compromised of a point man and and at very least, 4 other riflemen. If someone has the resources to be a support gunner or a DM (designated marksman) then they can be great additions to the squad. However, a squad should be based off of riflemen, as they are the workhorse of the squad.
A Sniper team is also a great addition to the team. If available, a spotter that can assist the sniper may prove very useful, but could also prove unwieldy, cumbersome and distracting. It really depends on the sniper. A sniper team will never exist of more than two men. Most of the time, snipers will go alone. They'll have to be very good at what they do, as airsoft sniper rifles do not have the same capabilities as real steal sniper rifles do. Real sniper rifles can hit ranges up to and beyond 3 miles. A good sniper rifle can hit around 300 feet.
It is very rare for a team to have a heavy weapons guy. Most of the time, riflemen will carry their own explosives, such as such smoke, and grenades. A lot of people also mount grenade launchers on their rifles. For starting teams, do not worry about this role.
Here's your first guide and piece of advice. Remember, KEEP IT SIMPLE STUPID!