How to if performing with an amateur group damage your professional reputation
Should a professional actor perform with an amateur theatre group? Except in certain specific circumstances I think it should be avoided. Amongst professionals, there is a huge prejudice against amateur actors in this country. In the USA it is quite common to find pro-am productions but it is regarded with suspicion here. This may be unfair (there are some very good amateur actors) but the prejudice exists. If you take a role with an amateur group you may limit your availability for professional work (what if an audition clashes with a Dress Rehearsal?)and it will be the kiss of death for your reputation amongst many people on the professional side.
You may long to play Hamlet but if you do it with an amateur group it will not be a professional credit on your cv.
I would also wonder how much one would learn in an amateur show. I don't think many amateur companies explore a text in the way one would expect in a professional rehearsal room. The approach is far more directive.
The exception to this is when an amateur company calls on you to assist them with your professional skills as a fight director, or choreographer etc. If you do this you should expect a fee. You should also be certain about the level of commitment in the group: dealing with lateness and absenteeism can be very frustrating. I have worked as director and voice coach with a very well-organised amateur company touring the USA and the Far East. I was well paid, well treated and found it a really rewarding experience, but I think that is quite different from playing a coveted part in your local amateur theatre.
If the profession is not giving you what you want then leave it and act with amateur companies. You will probably re-discover your love of acting and you won't depend on it to make a living.