It varies for team to team, but I don't think there is a specific "skill level" you need to be at to join. If anything, include that you are still learning (we're all still learning to a certain degree) in your app, and say that you want join to team to learn to learn from them.
There are dozens of benefits to joining a team, but they vary from team to team and person to person. For me, the biggest benefit to joining NEMESIS has been the knowledge I've gained from my teammates. I've learned more about drifting in the month I've been on the team than I did in the 6 months I'd been drifting on my own. Everything from help with tuning to learning new and better lines around various tracks.
Another big benefit is being able to drift with the same people consistently (this can be done without a team, but is more rare I think). You start to learn how the other people drift, and you can anticipate things on the track quicker. Drifting with the same people also provides a more consistant measuring stick for improving tunes.
I think most peolple that are on a team will agree that teammates tend to push each other to become better, and everyone on the team progresses and improves because of the friendly compitition between teammates.