Etsy teams are a great way to add more exposure to your products and your shop. In general everyone on a team works together to promote themselves and each other. Most teams you need to help promote others and they will help promote you as well.
There is many different types of teams on Etsy. Some focus on making treasuries and getting them to the front page so you can get tons of traffic. Other focus on posting your listings up on their threads for other prospective buyers to see.
Some teams are open for everyone to join and you may have to answer a couple simple questions to get in like: Are you willing to help promote other items as well as your own? If not then you might be better off on a different team that doesn't require that to join. Other teams are for members only and you must get an invite to have a chance at getting in.
Because a lot of the exposure you get on Etsy is how much work you put into advertising your product, teams are a good way to get a lot more views on your items. You must follow the rules and each team is different but they are for the most part pretty easy and their is no limit of how many you can join. Right now I am at 101 teams and post my items on them regularly. I started my own team about a week ago and it seems to be going good. Anyone can join so if your looking for a friendly Etsy team to join then stop by and check out mine! https://www.etsy.com/teams/20005/bold-promotions