Neighbor Hiking/dirt Biking Permissive Use Risk?

My sister and brother in law have 30 acres in California (el dorado county). There is a private dirt road on their property that one neighbor has been hiking on (she says the previous homeowners allowed this - or at least she has implied this by saying she “knew them”) and another neighbor rides his dirt bike. The second neighbor is now bringing friends to ride dirt bikes.
My brother in law doesn’t like having other people on the property but generally sees this as a low risk annoyance. I am concerned that he may unknowingly get themselves into some sort of easement (?) situation.
Does this seem risky to anyone else?
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