Groups or individuals can adopt monuments, makers, fences, buildings, etc in the park. By adopting you agree to come out 2x per year and maintain the site. Paint, rake leaves,pull weeds, etc. Adopters do not perform work on the actual makers or monuments themselves just the grounds around them. The agreement is two years atleast. They allow you to come out in the spring and fall. The maint. office at the park has a list of availabe sites.
Here is the link and contact info at maint.
http://www.nps.gov/gett/supportyourpark/adopt-a-position.htm