set newpost_minsecurity friends
You can also set the minimum security to "private" by entering:
set newpost_minsecurity private
You can turn off the minimum security level by entering:
set newpost_minsecurity public
If you maintain a community, you can set the minimum security level to "friends only" by entering in:
set for communityname newpost_minsecurity friends
One more thing: This does NOT change the security level of your existing entries. You can't make your journal all friends-only with this command. Perhaps you'll be able to in the future, but not right now.