Workspaces and plans are two different things. When you invite someone to your team, they are given the benefits you paid for, which include the following for each member (taken from the pricing page):
∞ SSH Workspaces
∞ Community Workspaces
10 Hot Workspaces
Premium E-mail Support```
In this case, it's simply a plan designed so not every user has to sign up for premium, and so that sharing workspaces with your teammates can be easy. As far as I'm aware, it's possible for each member to either create a workspace on the team account, or a personal workspace that they have on their specific account.
I am interested in why premium support didn't work, as according to the above features, it should be part of that. @moderators can someone weigh in?
Beyond that, I don't know a whole lot about the team plan. Hopefully someone else has some more experience with it.