You can say "wejdź!" or "wchodź!". To someone you do not know well you'd say "Proszę wejść". Whereas if you want to say: "oh, come on!" then I'd say "daj spokój!" or "no weź!" or "no proszę cię!" but these are rather individual things ;) If your girlfriend is outside and you are at home then my answer was correct. Otherwise, please advise ;)

Hmm I would go with "chodź" albo "no chodź" w tym przypadku :-)

Chodź :)