Once your team's been formed, members are then invited to join. You can do that here in the members area, or on the dedicated team page. Each member that accepts your invite will occupy a single seat on your plan. So if you have a plan with 20 seats and 5 members accepted invites, you will have 15 seats open. Members can belong to multiple teams yet you will just be billed for a single seat for each member.
If you invited a user and wish to cancel it, simply visit the members area or team page and cancel the invitation. You can also resend the invite if the user hasn't responded in 7 days.
If you need to remove a member from a specific team, visit the team page and click into the team you wish to edit. Select the member and choose 'Remove'. You will then need to confirm in a pop-up whether you want to remove the member from just this team or delete them entirely your account. What's the difference? If you remove the member just from one team, they can still belong to other teams you own. If you delete the member, that member will be removed from all the teams they belong to and you will free up a seat on your plan.
Note: you can also quickly delete members from your account by going to the members area and removing the member there.