Let coworkers join automatically
People with a matching company email can find and join your org without an invite.
This is on by default. If someone signs up with an email at your company's domain, they can find your organization and join it rather than accidentally starting a second one.
Organization · Joining
It only ever applies to real company domains. Personal email domains are never joinable this way, so nobody joins your org just because they use the same email provider.
NoteAn admin can switch it off in Settings if you'd rather approve every person by invite.
Still stuck? Ask your Chief of Staff, or contact support.