The maximum number of groups you can belong to has just jumped from 42 to 60! That's a very unexpected surprise present from LL. Because the number of allowed groups increased the load on the servers exponentially, tech-savvy SL residents all said it would most likely never happen. Now it has...but only if you are a Premium member. This is a very nice addition to the Premium benefits package.
Here's the official announcement: 60 Groups for Premium Members
And you read it here first!