In my opinion this is the most common question I've been asked by my friends, and it has been hard to answer but I will say this...
First off start by talking to him (this is if he's like a friend of a friend) and start making connections like: "so you and my friend know each other how long..." and start talking about who you are and ask him but dont ask too much! He'll feel clostrophobic.
I've been through this and it has worked, my current boyfriend was my best friend's boyfriend's best mate (confusing I know) but with a distant connection like that even bought us together.
Just start things small and make them grow as you go along.