Asking for on site talk at this point? At this point, most US PhD programs have made decisions for many students and are considering a few.

I will have the opportunity to be at a university these days. Will it be a good idea to email a potential adviser who interviewed me and ask him to talk for a few minutes with him and members of the research group since, if accepted, I won't be able to visit at the visit days?

They have yet to decide about me - and since I wasn't accepted on the 1st wave I am getting a bit nervous...
All you can do is ask; the best outcome is they say yes, hear your talk and accept you. The worst outcome is they decline to meet you and then reject you, which is the same as if you don't ask and get rejected anyway. I don't think they're likely to reject you based on you asking politely to meet them.

Apart from that, my only advice is to try not to worry. I remember how stressful the PhD application process is. As they say, good things come to those who wait, but I'd add that good things come sooner to those who are proactive. Good luck!