You probably already know most of the information about the contest, but here it is again, just in case:

HIM is asking it’s fans from around the world to help them set the record for the largest global Rock Band performance in history. Just make a video of yourself performing "Heartkiller" on Rock Band and upload it to the HIM’s YouTube Channel and tag the video “Official HIM Rock Band Entry” on March 25th at exactly 10 p.m. ET and you will be entered into an official contest in which the members of HIM will choose the Top 5 best performance videos. The band are asking fans to film their performances and post them at the same time in order to set the world record for the largest global Rock Band performance ever!"

It seems the guys may be playing XBOX live and you can join them if you have a Gold Account. Follow the link and also visit the official Heartagram site to find out more.