The user account created during Overseerr setup is the "Owner" account, which cannot be deleted or modified by other users. This account's credentials are used to authenticate with Plex.
There are currently two methods to add users to Overseerr: importing Plex users and creating "local users." All new users are created with the default permissions defined in Settings → Users.
Importing Plex Users
Clicking the Import Plex Users button on the User List page will fetch the list of users with access to the Plex server from plex.tv, and add them to Overseerr automatically.
Importing Plex users is not required, however. Any user with access to the Plex server can log in to Overseerr even if they have not been imported, and will be assigned the configured default permissions upon their first login.
Creating Local Users
If you would like to grant Overseerr access to a user who doesn't have their own Plex account and/or access to the Plex server, you can manually add them by clicking the Create Local User button.
Enter a valid email address at which the user can receive messages pertaining to their account and other notifications. The email address currently cannot be modified after the account is created.