You can integrate CTFd with Major League Cyber as an OAuth provider.
To start off, create a new event on MajorLeagueCyber by following the event creation wizard. Be sure to select the User Mode for your event to be the same as your settings in CTFd.
On your event page, click the "Edit" button in order to get your integration details.
Click on the "Integration" tab to get your details. In order to get OAuth with your CTFd instance working, fill in the redirect URL with your CTF URL + /redirect.
In the config page of CTFd, be sure to enter the client ID and client secret from Major League Cyber.
Now, people will be able to log in to CTFd using Major League Cyber!