2. Keep eye on the Event Calendar for upcoming organized Build Days. (usually scheduled 2 weeks in advance...)
3. Or, check the Messageboard for announcements about trail managers "heading out to work on trails tonight" type ad-hoc build (and ride) opportunities.
Basically, someone is not going to tap you on the shoulder and ask you to come help... but you WILL see requests for help - it's up to you to step up and say "hey! when/where!?"
There are many other areas we need volunteers to help with - besides building trails - we also need help with day-to-day organizational stuff or volunteers to handle special projects. Check the "Wish List " for areas we need help with right now (but that's just a partial list!)