How to get feed from wpmu site to my blog?

I want to get feed from wpmu site to my own wp blog. Addresses of wpmu site are http://user1.site.com, http://user2.site.com, http://user3.site.com and so on.

If I put main address http://site.com the feed doesn’t show anything. If I put http://user1.site.com then I will get only feed from user1.

What address I should put in rss feed plugin or is there any plugin which can solve my problem.

[moved from How-to to Multisite]