As far as I know there is not a direct search function, however, anyone you're looking for has most likely posted something, so, you can use the normal Search
feature. Type in their username, expand to 'By Username' and 'All Forums' and 'All Posts' and do a search.
You can also add people to your address book so you know when they're online or not - just go to their profile and click "Add to address book". I'm not certain, but I don't believe this overrides their hide/show settings... i.e. if they're set to have their status hidden, it doesn't show.