How should I talk to an unwelcoming future research group colleague so no interpersonal issues raise?
From your description, I'd say it's not time to worry yet:


Meanings convey poorly via words. See e.g. this "joke" that turned out to grievously hurt someone. You can't be sure your former classmate doesn't welcome you ... yet.
It's possible you're asking your former classmate the wrong things. For example you're asking about paperwork. Why should he know the paperwork better than, say, the professor or the department receptionists (or you for that matter, I'd expect the paperwork needed to be on the website)? He could be annoyed you're asking him questions you should ask someone else, which should resolve itself if you stop doing that.
He might simply be going through a bad period. For example if he just broke up with his wife and is fighting over the divorce agreement, you can't expect him to be very enthusiastic about anything.


If you're keen to join the group, I'd say join it first and only start worrying if it really doesn't work out.