Relevanssi Premium can search user profiles. So far the following data gets indexed from user profiles: first name, last name, display name and description. If you’re interested in having something else indexed, there’s an option on the options page.
Because WordPress only handles posts in the search result pages, Relevanssi must make WordPress believe it’s getting posts, even though it gets user profiles. This causes some problems, which are fortunately fairly easy to fix with some changes to search results templates.
Some problems are already taken care of. Relevanssi contains a filter that should fix
the_permalink() for user profiles.
It’s easy to recognize the user profiles. You can test the value of
$post->post_type, it’s ‘user’ for user profiles.
To make things slightly easier, there’s a filter hook. You can add a filter to
relevanssi_user_profile_to_post to make changes to the post object created from the user profile. It has the basic values already set.