Not easy to do, it would require access to the database and a good understanding of the schema to import the users. I have done this for a few of my clients in the past, pulling profiles/info and photos off of other sites and creating dumby users.
Depends what you are looking to do, if you are only looking to create the feeling of a popular site this would work, but keep in mind that this sort of work is usually custom and doesnt come cheap.
Are they fake profiles or real profiles? If real, are they exclusive or not?
Once that's sorted out just do a DB import - charge your hourly consulting rate for the import plus the cost of leads.
90% Real profiles and a few fake ones. In terms of exclusivity, 50/50.
Where can I find the DB to import? How?
Thanks