I'm a member of both AOPA UK and AOPA US because I fly in both. All of the AOPA's are members of IAOPA and work in concert. This is particularly true in Europe, where EASA is imposing common standards and lobbying at the European rather than National level is essential. By joining AOPA Germany you would be supporting that lobbying effort. While AOPA US has a clear interest in N Reg operations in Europe it is IAOPA Europe that is doing the grunt work.