To change your team size, navigate to the "Account" area of your SendBoard settings. Under the "Plan Details" section, you will see what plan you are on, how many users are included in the plan, and how many Trello users have been allocated to the plan.
Your team size refers to how many users are included in your plan. This is the number of users or 'seats' that you will be billed for.
Allocated users refers to the number of Trello users who are currently authorised users of SendBoard. You can't have more users that your plan allows.
Increase team size
If you want to add new users to your team, you'll need to ensure that your plan includes enough seats. If you are going to add several new users, set your team size to the total number of users you need. You can do this from the 'Manage Plan" area of SendBoard Settings: click on "Manage users" and then "Change Team Size".
Decrease team size
You can decrease your team size in a similar way. Just ensure that the number of allocated users does not exceed your desired team size. You may need to "Remove" users from your team before reducing the team size.