The more group members have in common, the easier it is for them to develop bonds.
However, in today's world, there are so many ways you can categorize people. People of the same race may have vastly different attitudes and values, for instance.
But more than anything else, people bond over shared experiences. Also, people with different mental conditions may have great difficulty relating to each other, even if they share the same culture or race. Thus, it is important that a group be focused on a specific mental health condition.