Click your workspace name in the top-left corner and select Plan & Billing from the dropdown.

Here you'll see how many seats are in your plan and how many are in use.

Click Seats in use to see all workspace members and how your seats are allocated.

If you're in the free trial or Standby this will appear on the left side:

If you're on a Pro plan you'll see this option on the right side:

On the right side of the screen, click Update seats and select how many additional seats you'd like to add. The updated invoice total will be shown on screen.

Click Continue to review the prorated amount due today and the updated total for your next invoice.

If you have a promotion code, enter it now. Then click Subscribe and pay to confirm. The new seats are added to your workspace instantly.

Head to the Team section and invite new members to fill your new seats.

That's it! Your new seats are active. Manage them anytime from Plan & Billing.