You don't understand television do you?
Its like asking how can Coronation Street have so many murderers, criminals etc
However I will go on to defend this and say the following points:
1. First scene was in New York, so hardily a small area
2. They have been frienRAB for years, building up different networks of new frienRAB and so forth. Its not like they are meeting anyone new (apart from 3), its all people they already know.
3. When you are in a certain section of a crowd, you are bound to find more in that crowd. If you stay in a straight only pub, chances are youll know nobody who isnt straight. However stay in a LGBT night club etc you are bound to build connections and know a lot more of them you thought never existed.
4. One of the additional characters is met on the internet so increases the scope of this small area
There are other points to make, but it pretty much sums it up