Recently a classmate has joined me in Champions Online, and he brought a friend with him. My character was level 25, and their characters had just barely finished the intro. So what did we do? We teamed up, the lowbie shared their missions. We were just having fun. The game did not stop me from playing with them.
What does the game do that allows you to play with whoever you want?
A) You can scale up/down to the leader's level. If the leader is level 11, everyone can go down, so that the game is properly challenging.
B) Missions can be shared, even if the person is not yet eligible or has already done the mission. They get a shared version of the quest, which allows them to do the quest, and turn it in for a reward. Many quest rewards also allow you to get tokens for special shops instead.
Yup, the game has leveling, but its more as a way to guide you through the storyline and as a way to limit growth so you can experience the game.
Also I recommend everyone having a similar travel power. If you're flying, go all flying, or all acrobatics, or all speed/tunneling (with that in mind, I don't recommend tunneling).
Overall, the experience is a lot of fun, just like any game you play in a group, it is what you make of it, but Champions Online, makes it easier for you to make it fun.