Login As User from Frontend In the CP there is a button which allows you to log in as a specific user. Can this be called from the Frontend/plugin or is it strictly a CP function?
You can call it from the front-end, but it requires the currently logged in user to be an administrator.

Just point the form's hidden action input to users/impersonate and have it submit a 'userId' input of the person you want to log in as.