you will only be able to interact with people from the group you are in or the group you are going to. Thus, if I am in the GIT, I will not be able to interact with anyone from the respiratory tract, as it would take me two days to get there. I can however interact with anyone from the GIT, or anyone from the blood stream.
Another mechanic is that you can interact with anyone in your group, except when you are in the blood stream. While you are in the blood stream, you will not be able to pm with anyone until you enter another group (unless your role specifically states otherwise).