You can subscribe to users on a different StatusNet-based server to the one where your account is. For example, if you are using identi.ca under the name of 'evan', and a friend is using example.com, you can subscribe to them by doing the following:
- Visit their profile page on the remote server, e.g. http://example.com/myfriend
- Click 'subscribe'
- Enter your profile url, e.g. 'http://identi.ca/evan'
- Click subscribe.
You should now receive their notices in the usual way.
(See more info on openmicroblogging at http://identi.ca/doc/openmublog)