If using the park bus system does the senior pass still apply for all in the party ?

Two of us went in last week. My senior pass worked for both of us.

I'm not sure if I understand which bus system you are referring to but both the Springdale bus to the visitor center and the bus from there was free (provided you paid to enter the park, of course).

It's only valid for up to 4 adults and children under 15 are free.

I don't know. We went by car...