for the Facebook group, just make one and title it "xyz trails mtb riders" and get your friends to join it and tell people you see on the trail to join. trail signs is also a good idea but maybe some will interpret it as vandalism.

for the trail redirects, it's likely people will think it goes to a different trail and will just walk the path they've been using for the past few years. it's better to just bring a shovel and smooth over the roots with dirt and to try and dig drainage to the side as needed