Settings → Members.
Members appear in the Members table with their role. Owners can change a member's role or remove them from the row menu.
Seats
Plans include a number of team members (Starter 3, Pro 10, Enterprise unlimited). Pending invitations do not count until accepted. The invite button explains when you are at the limit.
The invitee says the link doesn't work. Invitations expire after 7 days — resend. They must sign up or sign in with the same email address the invitation was sent to.
Can one person belong to several workspaces? Yes. They switch workspace from the account menu.
Transfer ownership. Only the current owner can do this, from the member's row menu → Transfer ownership.