Correct me if I’m wrong, but are you looking to send out bulk invitations (first time emails) to members on your database? The question is how you are setting up signing in… SSO? Local login? By Invite only?
You can send bulk invitations through the Admin Dashboard. Click on Users, then Send Invites:
Then click Bulk Invite from File. The file has to be cvs file. The link below this screenshot tells you what is needed and where to put it in your file.