"Alle sind bisher willkommen."
German often uses present for future, but not for past. "Have been here" is not a past tense in English either, it is present perfect. It tells about something that started in the past and is still going on. In German (and I think in many other languages including my native Russian) you don't care when something started, what is important is that it is still going on, that is why you just use present tense.
"All were welcome up to now" is clearly past. They were welcome but they are not now. This is not what the German sentence means.