When you are in heaven can you still see people?

Nope everyone is invisible.

Seriously, it all depends on what religion you believe in. Most Christianity based religions will tell you yes, anyone who has made it to heaven you will be able to see. However, some state you only see who you want to out of those people because you create your own heaven. Some other religions don't believe in heaven but an endless cycle of life, when you die you get reincarnated. if you were bad you will be reincarnated as something lesser.

A scientific approach to religion (admitting that heaven existed) would say that your mind would create and populate your heaven.

Pretty deep topic if you want to get into it.