US properties typically do not have extra charge for 2nd person, some Europe and Asia hotels also do not have extra charge for second person (Paris has extra city tax since it's per person). But quite a number of Europe and Asia hotels do charge extra (higher room rate) for 2nd person.
GH Taipei has higher rate for 2nd person and I was not allowed to bring a guest into GC without paying. GH Bangkok did not care when I had 3 people in the suite and all 3 had GC access and breakfast in the restaurant. PH Siem Reap and GH Bali also did not charge for 2nd/3rd person and all 3 had free breakfast.