At 18:00 (so in less than half an hour) I'm judging a prerelease. However, at this moment, I have no participants. What do I do if I get less than 8 players? Quick responses are greatly appreciated.

From the MTR:

10.1 Participation Minimums

Participation minimums for a tournament to be sanctioned as a rated tournament are as follows:

• For individual tournaments, a minimum of eight (8) players must participate.
  
• For team and Two-Headed Giant tournaments, a minimum of four (4) teams must participate.

If the participation minimum is not met, the tournament is no longer DCI-sanctioned and will not provide Planeswalker Points. If participation minimums are not met for any DCI-sanctioned tournament, the Tournament Organizer should report the tournament as “Did Not Occur.”

Basically, if you have less than 8 players, you can't run the event.

Sanctioned events require 8 players; unsanctioned do not. I believe that you must sanction events in order to use prerelease kits (that should be in your paperwork) - i.e., any use that doesn't exactly match 1:1 with sanctioned participants is a violation. That's to avoid any sort of “sales” of new product before the release date, so I know that's something Wizards has always taken seriously.

If you can, contact your local WPN rep or someone in the Wizards' EU office, to see if you can get an exception. Lacking that, I would not even consider running that event.

Hope this helps; I'll admit, I haven't run a prerelease for a number of years, but I'm fairly certain that Wizards is still very concerned about proper use of that product, and preventing sales prior to next Friday.

d:^D

Issue resolved, I'm at 11 players now, including 3 people who are apparently called comma :D