Does your company have a recip agreement with them?
If so, there should be procedures.
I once tried to JS on a 135 outfit which we had a recip with (my buddy was the pilot). After several phones calls and confusion on the part of the ops folks they learned that their insurance did not permit pax unless they were company employees on business.
That recip agreement was rescinded, as it should have been...many or most 121/135 recip agreements are pretty one-sided. In fact some 135 outfits use 121 JS's to re-position their pilots (clear violation of the rules).