Semaphor account admins can whitelist domains so that any join requests from those domains are automatically accepted. In the Semaphor app click the Channels icon at the bottom-left of the application window, click on the three dots next to your team name, then select Manage Team. In the Manage Team window, select Permissions. In the box under Auto Accept Join Requests, enter a domain name or series of domain names, separated by commas.
Once the domain names have been entered, join requests from those domains will automatically be accepted.