Tactics depend a lot on the powers of those you’re teaming with. C.E.C.I.L. teams seem to be fairly laidback (compared to some pickup teams I’ve been on). Just stick with the team and go with the flow.
In general, a team should not aggro more enemies than it handle. Usually this means someone should snipe or taunt a badguy (the pull). Often more than one will come. If it’s too many, then everyone runs. Otherwise, don’t attack the pull until it’s outside aggro range of its non-aggro’d friends.
Let the tanks and scrappers have the most aggro. That means others should mostly attack the badguys who are currently being attacked by the tanks and scrappers. It’s better to focus attacks on a single badguy to kill it faster. Kill the bosses and lieutenants first. They do the most damage per second and are most likely to kill a hero. They also have powers (holds, stuns, raise zombie, etc.).
Area of effect attacks are tricky. They are very helpful, but can be really dangerous for picking up aggro. Be ready to escape. Which brings up another hint: if you have to escape, don’t use hover! It’s too slow. Drop to the ground and sprint.
Don’t attack badguys who have been immobilized by a controller (unless it’s a boss). Concentrate on taking out the enemies who are actively hurting your teammates. If you notice a teammate is hurt bad (keep an eye on the team list), attack whoever is attacking him. This will distract the badguy and give your friend a chance to escape.
If you’re hurt bad (your health bar turns yellow or red) disengage! Do whatever the team’s healer needs you to do to be healed easily. You’re no use to the team when you’re dead. You can somtimes run to the mission exit, leave and come back for a nice heal.
Don’t stray far from the team. Many defenders can buff their teammates, but only when they’re close together. Plus there’s safety in numbers.
Finally, know your own powers. If you think you could use them more effectively if the team tried a different tactic, let us know. Communication is the key to a fun and effective team.