In NH it works the same way as @mreaves and @dryslot said. The local club should me more than willing to work with you on a trail reroute on your property. how often that it is used totally depends on the location and where the trail goes. as far as summertime use, in NH unless land is posted, people can use it legally. the snowmobile club won't have a say in that, but your neighbors might use it. If you don't want any motorized vehicles (dirt bikes, ATVs, etc), then you will need to post it to restrict that use. that would be enforced by local law enforcement.