I’ve a feeling this will become much more useful as time goes along…the following is taken from the LJ FAQ and is therefore more specific for LiveJournal (and its clones); if any of the links don’t work correctly, I’d replace whatever journal site pops up with “livejournal.com” and reload from that.
FAQ Question #232
How do I let someone from another site read my Friends-Only entries?
Once they have posted a comment and established an OpenID account, you can click on the OpenID logo next to their comment to reach their User Info page. Friend them with the Add Friend button in the toolbar at the top of this page.
After you have added the person to your Friends list, they will be able to read your Friends-only entries while logged into LiveJournal, both on the LiveJournal site and in your journal’s RSS feed. They may find the RSS option convenient if they do not frequently access the site.