It depends on the trip, where it is and what you are doing tbh. Also depends on how many other people are going.
I work in Y1. Our last trip involved us splitting into 3 groups of 10. Each group had a member of staff and one or two parents per group. We try to avoid leaving a parent with a group of children on their own if we can. If we do need to then we will put two parents together, and we carefully select children for each group also.
Typical things we ask our parents to do:
If walking - walk on outside of pavement, nearest to road
Hep with road crossing - may be standing in middle of road to ask cars to stop, etc.
Accompanying a group as an extra pair of eyes/ears
If with a group with no teacher - regular checking group numbers, etc.
We ask our helpers to wear a hi vis vest - sorry!
We tend to have a member of staff to do formal number checks, registers, and toilet visits.
All parent helpers are given a copy of the risk assessment for that trip.
We don't give parent helpers the job of carrying or administering medicine, inhalers, etc.
Is that it???