My nikkor 85mm f/1.4 ais has a stiff focusing ring?

Is it harming it to continue using it? And is there a website depicting the regreasing process?

If it's an old lens then it simply needs to be cleaned and lubricated. Grease hardens in time. If there's a grinding feeling to it, stop using. Something's terribly wrong inside and it may be parts out of place due from a fall or hard contact. Either way, have your lens serviced by a knowledgeable technician.