Tailscale networks are based on your email address domain name.
If you signed up as email@example.com , only users with an @example.com email address are able to access your network. Users who sign up with an @example.com address will automatically be added to your network. For most companies this makes setup easy, since your whole team can sign up with their existing work email address.
To invite others to your Tailscale network, ask them to sign up with your team’s email address at tailscale.com/start.