How to assign a user to all blogs on multisite

Is there a way to quickly add an existing user to all sites on a multisite install? Right now, I have to go to each blog to add the user which gets painful since I have more than 20 sites on my multisite network.

E.g. RootDomain.com is the root install with username1 as the administrator. I’d like to add username2 as an editor to the sites MultisiteDomain1.com, MultisiteDomain2.com, etc. in one action, instead of having to add username2 to each site individually.

Thanks.
Ben.