Aristotle had it right when he wrote in Nichomachean Ethics that your closest friends are: 1. your peers (e.g. guys you cannot gain from financially in terms of getting a promotion and vice versa); 2. guys you have known for a long-time; and 3. guys with whom you have shared salt (e.g. shared some difficult experience with - perhaps served with them in Iraq).  I believe these characteristics are largely true today. 

In Buddy System: Understanding male friendships, men reported that friendships included trust and dependability.  So if you are trying to groom closer friendships, start by looking at the above categories of friendship, think where you are with your friends, and decide if you want the trust friends to become the must friends.  Use these simple rhyming categories with your clients if they are seeking closer relationships or feel disconnected from others.  Engage men by being trustworthy and dependable - do what you say you will do and don't share intimate details about your friends with others.  "Get their back" when they are not around, or even if they are around.  Friends protect each other. Finally, find a level of openness that feels comfortable to you.  Some sharing of information and feelings must occur for people to become closer with each other.