I can imagine that there are many professional techs that started out by working on amateur and school production. And I imagine that a reasonable percentage of them still have some involvement with amateur companies. As they feel like they should offer something back to the group(s) that started them on their career path. but there can be a danger in this, some would see it as selling your skills short. But others think of it as passing on knowledge to those others that are keen to learn.
But there is something that will rear its ugly head, the possibility that you will be taken advantage of. This is very common, I know because it has happened to me. The problem is that you have these wonderful skills that are sometimes more developed than your amateur counterpart. So you keep being asked to do this or that, and you feel obliged to help out. They then assume you have lots of time to give even more help and soon it snowballs. So you need to set some guidelines for you and them. You have to say to them I am willing to undertake this task and spend x number of hours on it. Also remind them occasionally that you do this for a living. They also need to acknowledge what you are doing for them. This is a situation of give and take with the results being mutually beneficial to all concerned.
Do not be afraid to pass on your knowledge. You may occasionally come a cross someone that might resent you being involved. This might be due to the fact that they have been there for ever etc., this person you do need to get on your side, otherwise things will be difficult and there is going to be tension within the group. The thing to remember is that you are all there to have fun and put on a great show. You need to remember that these people are doing it for the love, there is no paycheck at the end of the week. Just the satisfaction of doing a well done job to the best of their abilities. You can also learn a lot from working with these types of groups. Tolerance for people with less skills than you is one. It will make your job easier when an amateur group comes into to a venue that you work at. You have a better understanding of a community theatres group dynamics and the fact they have less time and money to achieve a professional job.
So if you decide to help out at an amateur theatre group, enjoy yourself. freely pass on skills and enjoy the fun as well.