Team member limits by plan:
Free — 1 user (no additional team members)
Starter — up to 3 team members (including the account owner)
Pro — up to 10 team members
Enterprise — unlimited team members
If you need to add more team members than your plan allows, you will need to upgrade. Go to Billing → Manage Subscription to compare plans and upgrade.
Pending invitations (invites that have not yet been accepted) still count towards your team member limit.