I am building a social networking site that will allow users to add themselves in group. Now, I am in dilemma to allow users to create groups or not.

* Single group concept - StackExchange, Quora etc. 

**Example**: For a single category there is one group ex: C# programming. Users are not allowed to create any group on the top of it. They can post their queries under this group (a.k.a tag, category)
* Multiple group concept - Facebook, Google+, etc. 

**Example**: For a single category (ex: C# programming) users can create multiple group ex: Houston C# programming, Texas c# programming, Yet another Houston C# programming.

Now, the question I have related to it:

* Do users feel comfortable joining multiple groups for the same category?
* What is the benefit of choosing one concept over other?
* Is there any criteria for choosing the concept?