I think the majority of .net developers do not attend user group meetings and do most of their research and learning online or in formal classes. Of all of the .net developers I know (well over one hundred or more), only 2 belong to user groups. I don't have actual numbers, but I do know that in some cities (Boston, Orlando, Atlanta, San Diego, and even Philadelphia), you'll find a higher % of .NET developers who have attended at least one UG meeting per year (even if they are not a regular member). User Group attendance in certain other regions (Baltimore, D.C, and Northern Virginia) has sky-rocketed.
In the larger cities it's a bit more likely to have larger development teams, and so if one person attends, odds are that person will get others to attend at least once in a while.
In our instance, our training facility hosts the Northern NJ .NET/SQL Server user group meetings.