Hello Tamodachi
I dont understand what you mean by a team? Do you mean the group and teams for chess.com. If you mean this, it is not necessary for people to join these. Usually people join these to talk with other members, to join team matches, or play vote chess with the other team members.
It is also not necissary to get a premium membership. Premium Membership gives you a few extra things you get access to. For example, gold membership cost about $20 a month I think. This membership allows you to make your own team/groups, but this is the most basic. You get to join more live chess and online chess tournaments as well as get more of the learning options. These learning options are tactics trainer, chess mentor, etc. With other memberships such as platinum and diamond, you are able to get more and more of these bonuses.
http://www.chess.com/membership?c=icon near the middle of this page tells you all the bonuses that you get with each upgrade of membership.
Hope this helps:D
Hi. Could someone advise me who I set up a team please. Do I need a premium membership?