I'm a Director of AOPA UK and a member of US AOPA.
The overarching body is IAOPA, the International Council of Aircraft Owner & Pilot Associations, to which all national AOPAs are affilliated.
You should contribute to the AOPA covering where you fly. I am a member of both UK and US AOPA's because my life is currently split between UK and US and I fly in both places. The subscription is minimal and so are the overheads so little of the money that comes in goes on administration. My own work for AOPA UK is entirely voluntary, I don't take any fees or expenses. It's my way of giving something back to an activity that I have enjoyed for many years.
AOPA differs from many organisations in that its work goes to benefit all aviators, not just its members. In the US they recognisee this and the uptake of membership as a proportion of flyers is much higher than in Europe where a "what's in it for me" mentality pervades and many seem happy to sit on the sidelines and not support the work that's being done on their behalf.
Nowadays, when the European National CAA's no longer control the regulation of their own State's aviation activity it's essential that we have representation at European level. Like it or not that costs, even if it's just travel and accommodation.
Anyone who flies should contribute.