I've got a pokemon go game started with my google account and have been playing with my son. My wife wants to put the game on her phone too so she can play with him during the week. But on her phone (Google Android), it seems to force login with her own google account set up on her phone.
On mine (iOS), after hitting Sign In, it actually gives me a login box to enter email, and then password. On hers, it seems to be an automatic login and goes immediately to the new game setup. We can't find any option to say which account I want to log in with.