You are billed for active members of your workspace. Each one is a seat.

Who takes a seat

Anyone with an active account in Settings → Users.

Who does not

Client-portal users. Customers who see their own records through a portal are not workspace members and cost nothing.
People who never accepted their invite. An invite is not a seat until it is accepted.
Deactivated members. Deactivating frees the seat.

This matters when you are sizing a plan: a five-person agency with forty clients needs five seats, not forty-five.

Adding someone mid-cycle

Prorated — you pay for the part of the period they are actually there, not a full month for someone who joined on the 28th.

Removing someone

Deactivate them under Settings → Users. The seat is freed and can be reused.

Deactivating is not deleting: their past work, comments and history stay exactly where they are, so the record of what happened remains intact. What ends is their access and their seat.

Checking your count

The Users screen shows seats in use against your plan. Worth a look before renewal — accounts for people who left are the usual reason a bill is higher than expected.

Free plan limit

The Free plan is capped at 5 members. Beyond that you need Pro or above. The limit is on people, not on features: tasks, docs and core views are unlimited on Free.

Seats: adding and removing people