I think it should be more casual, that tends to be recieved better. Along the lines of “Hey all, Wanted to let you know that registrations for this year’s Flags on the 48 event have started. I just signed up for _______. For those who don’t know, FOT48 is a September 11th Memorial Hike in the NH Whites. If you want to know more, let me know, or check out the site at http://www.flagsonthe48.org“.
We should try to avoid posting a canned message to every site. The more personal it is… the better recieved it will be.