A team has guests as well as admins and members. A guest is on the team for only what they were given: an area as a whole, such as Documents, or only the records shared with them. They never see the team's other areas, its pages, its billing or its Trash, and they take no place on the plan.
Invite guests from the team's Members page by handle, email address or link, and choose what each can use. Personal can have guests too: open Guests in Account settings. The Members page lists guests under Guests, and a guest's page shows what they can use, the records they are in, and Remove.
Invitations wait on Your teams and in the app, with Join and Decline, and the person who invited you is emailed your answer. A team you are a guest of is labelled Guest in the Personal or team selector, and someone else's Personal is named after its owner. Free allows 10 guests per team and per Personal; Pro and the Team plan allow 100.