In reply to David Coley:
Both places are great, and both are ideal for a mixed-ability group... I'm sure you'll have a great trip whichever you decide. If I had to summarise the two...
Rjukan - more climbing options, so slightly better for a longer trip, particularly if you're just set on climbing. The ice is good (often very stepped out), but there are usually lots of people and I personally don't think the valley is the most attractive. Short drives to almost all of the climbing.
Lillehammer - In my view, a nicer valley. It's way more open and spread out, and gets more sunshine!... But the drive to the climbing will be much longer. Despite several large (and excellent) ski resorts, most of the climbing has a more remote and adventurous feel to it. Few of the routes will be stepped out, and there'll be a lot more breaking trail (so it depends if you like this kind of thing!) If you've got any non-climbers, or wish to combine with skiing, then Lillehammer wins it for me. There are, however, far fewer routes if you're there for a long trip.
You could always do a week in each!
Also consider Setesdal, which has slightly less reliable conditions, but is an amazing place when in good nick!