I can't say when Josh's website will be back, & unfortunately for you Room Filler relies on that site being up in order to function, I'd rather see an honest user count anyway, so I'm not desperately upset about this particular consequence, the site is missed for other reasons.
However, if you simply must fill your room with fake users (you might guess I personally disapprove, it seems dishonest, as well as being at least a form of spam), then grab Nushi 1.8 from the archive & enter it into the room, then without leaving hit the join button, enter a new name & enter again, repeat till you have enough fakes.
Be aware you'll have to paste the roomname in manually as 1.8 has no roomlist & you will also have to manually enter all the numbers with each name e.g. Fake_User123_12345 some rooms will prevent entry without numbers, others will truncate the name, & some rooms will require configuration to allow multiple joins from the same IP.