There is a add sidebars plugin at http://bp-dev.org/plugins
Install that and use text widgets on those sidebars to show ads.
I highly recommend openx, depending on the volume of your site, this will allow you to do amazing things. It is your own ad server, so you can control impressions, clicks, etc.
http://www.openx.org/
Yeah, openX is by far the best the best option – If you’re smart enough when setting it all up, you’ll be able to produce some fantastic results and generate invaluable learnings on user habits which you can then use in turn to make more cash.
If you run a number of sites, it really comes into it’s own – You’ll essentially have your own ad network with the ability to gather learnings from each of your sites – find out which ads generate the best CTR’s and as a result give you more money. It’s even better if you can get pixels placed on the confirmation pages for whoever you’re advertising as then you’ll be able to work back to an average eCPM.
Oh, and you can even track adsense clicks with it too. Highly recommended.
Try using ads plugins that provides widgets and install bpdev-widgets ( it’s included in bpdev plugins ) and put the widget in the users & groups page
(I know it will not help you, but it could be useful to know 
)
Thank you very much for being so kind and for your interest to tell me all the solutions.But if i will use openx, i will be able to use it for all the blogs they will be registered?If yes,can you tell me in big lines how can i do that if i have several templates for the users to choose from?
I m also wondering what is the best template to use for home theme?
You could try our Buddypress Ads Plugin available at https://wordpress.org/extend/plugins/buddypress-ads/
This plugin will allow you to publish ads through your buddypress site(with wordpress mu) Supports multiple ad networks including adsense , adbrite and many more. Just replace the ad code in the file bp-custom.php with your own ad code.
More details at http://www.pingsense.com/2009/11/buddypress-ads-plugin/
I was looking for an ad solution for BuddyPress everywhere and was disappointed to find the only solution discontinued. I developed a child theme for BuddyPress you can use to implement ads to the default BuddyPress theme and would like to share it with everyone. You can download it here: http://www.rizonesoft.com/more-downloads/bp-replenished/
