This would depend on how you have your adsense set up now, really. There is a limit to how many ads you can display per page, iirc it's 3 text/image ads and 2 link units (or something thereabouts).

Any ad space you give your members would take away from your page total, so just keep that in mind. For logged in users I only show 1-2 ad slots depending if they are contributors or not. If you do give a slot to your members, I'd make sure you set a fixed number of ads that they can run - perhaps limit it by character count or so. You don't want people to be able to just paste all 5 slots in a row into their blog.