D..
Yes, they can join a team. My kids had lessons at the Y, and my husband asked me to sign them up for a swim team. I was shocked and told him that I didn't think that they could swim well enough. He told me they would learn to swim well enough by being on a team. He was a swimmer in school, and he knew about this stuff. He was spot on with my kids, too.
Being a swim mom is a lot of work. But my kids had natural talent and they really excelled. The thing I liked so much is that swimming is a team sport, but you are really competing with yourself. People love to watch the younger kids swim because they are so "brave" - quite a feeling for the little kids for everyone to clap and ooh and ahh over them. And the bigger good swimmers are great mentors to the up and comers.
Go to a few practices. Watch how the coaches act. If you have several different teams, don't choose the most competitive. Don't choose one where the coach is a jerk. That doesn't mean choose an ineffective coach, though. That will do your kids no favors.
I was organized with my kids "stuff" but made them be part of the organizational process. I watched them swim at the practices (I didn't leave like some of the other moms, who had no clue what was going on), made it perfectly clear to them that they couldn't run around the shower and locker rooms (in fact, when they were just taking TOO long, which meant they were playing instead of showering and dressing, I no longer allowed them to shower. I took them home for that instead.)
My younger son is still swimming and this week was awarded MVP of his high school swim team. I am very proud of him. More than that, I'm glad that he has a lifelong love for being in water and being able to navigate in it. I am a terrible swimmer and didn't want that for my children. A swim team will make a big difference in 'your kids' ability to swim, and will make them decent athletes.
Good luck.