Currently, the only plans that support multi-seat Workspaces are the Scale, Business and Enterprise plans. It is not possible to add additional users to any of our other subscription plans.
On the Scale plan, each Workspace comes with 3 seats - one admin user and two additional users.
On the Business plan, each Workspace comes with 5 seats - one admin user and four additional users.
For more information, please see our Workspaces documentation.