If you open the app `I'm a WP7 and go to my Hive, swipe over to "About" and you will see that (emphasis mine)
A WP7 Hive is an arbitrary grouping of users that gets assigned
when you register your Windows Phone. What is the purpose of a Hive?
Well, honestly we don't know, and no one at Microsoft seems to
know, but we are lucky enough to have the secret formula to figure
it out....We are using this sneaky information to make it easier (and
more fun!) to connect with other Windows Phone users worldwide.
Now, that leads you to believe that it is something some developer at Microsoft implemented, and has since left leaving no one to know what they are. Although possible, I am too cynical to believe that.
What do I think it is?....The developer of the app made them up. Yup, sorry but I don't see anything on Bing/Google/WP sites/(or even asking some people on the WP team) that knows what they are LET ALONE heard of them.
I'm going to preface this next statement with a "This site is not for developers", but as a developer I have not even seen this information available in the API, and since this is an official app, I know it is only using legit means to do it.
So what are they? Well, I think the app is using them to
make it easier (and more fun!) to connect with other Windows Phone users worldwide...a gimmick
I could be wrong, but that is the only thing I could come up with. The only other possibility I can think of is they are actually called something else, and the app developer chose to use "Hive". That would explain why nothing comes up (except this thread!) in a search.