Your plan includes a set number of seats. When you outgrow that, you can add seats rather than upgrading the whole plan. [CONFIRM: seats included per plan and the price per extra seat]
- Open Account → My plan or Account → My team.
- Add the seats you need.
- Invite the new people into the seats you added.
If you regularly need far more seats, compare the cost against moving up a plan. Scale may work out cheaper than many add-ons.