On the Team page, Invite someone shows how many seats are used, for example “7 of 10 seats used”, and Seats shows how many you have and how many are left, for example “10 seats, 3 left: 5 with your plan and 5 more members.”
Add 5 members… adds a “5 more members” pack: $79/month, billed yearly ($948/year). The dialog shows the price first: what you pay now for the time left until your plan renews, and what the plan then renews at. Confirm with Pay and the amount.
When every seat is taken, the Team page says so and offers to add 5 more members. Add as many packs as you need.
Remove 5 members… takes a pack off. Its 5 seats go at once, so your team, waiting invitations included, has to fit in the seats left first. The plan then renews at the lower price, and nothing is refunded for the time left.
Swapping one person for another costs nothing: remove one, then invite the other.
If Seats shows Ask us for more seats instead, for example on a plan PanoCool set up for you, use it to ask PanoCool for more.