Not sure why you want to report them if they are being respectful & not hurting anyone or the land and if they are mowing & caring for the lot.
But if you want to - you can look up who owns the lot on your counties tax site & then try to find a phone # for the owner via the phone book. Unless you own the property - I don't think you can charge the guys for anything or call the police on them... unless they are don't something wrong.
There is an empty lot between 2 house I own - the guy that owns it owes a ton of money in back taxes & the city told us if we want to maintain the lot that we have the right to... we do plant a garden on it, mow it, put a fence along the front of it & planted a flower bed infront of the fence. Everyone in the neighborhood was happy to see it taken care of & all the flowers blooming. They are so tired of seening all the empty lots with knee high grass & trash. Plus, I was told by the city - that if the guy that "owns" the property doesn't pay his taxes soon - the city will take the lot for back taxes & sell 1/2 the lot to whom ever owns the house on both side... since I own them both - the lot would become mine for a very small price.
You never know - they may have premition to use the lot. Just as I do... oh btw - I tried for almost 2 years to locate the "owner" of the lot & he REALLY doesn't want found, cause the city & county can't find him either.