Is it possible to share / split up revenue with someone? For example.. I'd like to compentate someone who works on a forum but giving him 20% of ad revenue. Would this be possible directly with adsense.. Or is there some other way to do this? - maybe external software? Thanks in advance for your advice!
yes, this is allowed which called as adsense revenue sharing This link ight help you to know more details http://code.google.com/apis/adsense/revenuesharing.html
Yea, it can be done both externally or internally. since its about a forums revenue sharing i think their is a mod names "revenue sharing" which distributes the revenue as per the set percentage in the mod admin panel. the mod i am talking about was for S.M.F but i am sure you will find similar mods for any other bulletin board software as well.
Even if it's possible take caution, sharing your earnings with other people, could not be a good idea.
If you let other user contribute content to your site then you can share revenue with them. Many sites do that e.g. hubpages.