Sorry for the bold letters last post...
Here are some more links:

the camping, where you can also rent out bungalowtents for 8-10 people. This is by far the cheapest option I've found so far, but the Netherlands can be chilly in May.

The tourist centre in Groningen, with lots of information.

Hotels in Groningen.

I cannot find prices for Easjet flights, since the May 2003 flights aren't online yet.
Letty