Sitefinity provides a setting to configure if the backend authentication screen shows (or not) the drop-down to select the user groups.

Navigate to Administration -> Settings -> Advanced -> Security
At the bottom of the page, there is a setting Membership provider selection on login. When the selection is set to Auto, Sitefinity automatically detects and selects the user’s membership provider during authentication into the system’s backend. When in Manual, a dropdown list with user groups is visible on the backend login screen and users must manually select their user group (membership provider).

https://community.progress.com/s/article/user-groups-dropdown-is-unavailable-in-the-backend-login-screen