Yes, but you may find the speed/memory of our standard shared hosting won't be enough if you get a lot of traffic.
MultiSite doesn't really use more memory/CPU than Single Site, but for every site you run, you really are running another site. So if you have 10 sites on your Network, it's very much like running ten separate sites, when it comes to server usage. This means that while we do offer unlimited, you should read this: http://dreamhost.com/unlimited-policy/
The important part is here:
What we mean is, you don’t have to worry about disk storage or network transfer when your site gets popular.
You DO still have to worry about plenty of other things though… if your site isn’t well optimized and is causing issues for others on your shared server (either because it’s pounding the CPU, hogging RAM, or doing tons of disk I/O), you may be asked to sign up for your own DreamHost Private Server.
Trust us, every host out there has exactly the same limitations… we’re just trying to be up front about them!
BuddyPress networks have a tendency to hit those pain points faster than 'normal' WordPress sites, or even a normal Multisite.
Can you install it on Shared Hosting? Of course! :) You just may find yourself asked to move to a VPS if you get big.