As someone who is on the receiving end of RDA helpers as I carriage drive with an RDA group, there are many ways that our helpers help.
We have people who just make cups of tea and handround biscuits (they make the biscuits too - yum). We also have non-horsey people who cycle by the side of us to go to either our assistance or if the pony spooks at something one of them always has a leadrope. Then there are our co-drivers (Able-bodied Whips) who are able to drive and take over when necessary as they have another set of reins. These tend to be people that have been trained by the RDA to do that.
The riding RDA helpers are very similar people as some of them are not horsey or they feel that they are not confident so they do other things that help us. Or as rockyrobin says people that run or walk alongside. They also deal with any special equipment that needs to be added to the normal tack. Everyone gets special training on lifting and special needs requirements, although anyone that has specific medical needs always has a person accompany them from either their school/centre that knows them and how to deal with anything.
One thing I would say is that there are lots of different types of RDA riders/drivers, you would have to be willing to talk and help people with everything from severly mentally and physically handicapped people with speech or co-ordination problems or people with autism or other neurological conditions who say strange (sometimes rude) things or people like me who look and behave just like joe-normal until its time to get out of the carriage or dismount and find that their legs arent connected to their brain anymore. If you are not bothered by anything like that and can keep calm and composed and have a smile on your face and laugh with us and not at us, then you will be fine.
I would say that if you wanted to get back into horses, then its a brilliant way of doing it, all the horses that are chosen have to be nice friendly and calm. We do lots of different and fun activities, such as picnics and day trips which is brilliant which all the helpers get involved in. My RDA helpers have said that the best rewards they get is when someone achieves something, even the tiniest thing that they couldnt do before. For me it was being able to trot through cones at speed in the summer, they said that they could see the smile on my face from behind me! Driving is the absolute highlight of my week, and I couldnt get through some weeks without it thats how much it means. And for most RDA riders its the same. For some people its the only time they can be really mobile and for people in wheelchairs when they can be higher than everyone else (which actually means a lot when you spend your life looking at peoples middles).
Sorry to have a really lengthy post, but I felt that it was important to put the other side.
Have a great time - you would be really appreciated.