Invite someone to a project or to one conversation, and pick a role for them: Can view to read along, Can branch to try their own angle without touching your main thread, or Can edit for full access. They accept the invite before anything is shared.
Everything shared with you appears under Shared with me. Each person works from their own account and uses their own credits, so inviting a teammate never spends yours. A read-only public link is still there for sharing with anyone.