From what I’ve seen, you can’t enforce quotas on your own users. If they have the privileges to do the one click installs, etc, they also have access to disable any quotas. Anyhow…
First, you need to set up the appropriate Users and then Domains. Next, go into Billing and select Account Privileges. Give these people Domain-Only access.
As I said in another post, as long as you trust your friends, it shouldn’t be a problem. There’s a ton of disk space and bandwidth, so I’d be surprised if they’d max anything out.