Option A: use a rental/hire company who provides intermodulation & touring systems
Go to Sennheiser.com, choose your preferred language.
Under products, go to Wireless.
Under wireless, choose the type of product (G2 for IEMs, 3532 or 5000 series for RF mics for example) and once each page loads, it allows you to choose a country and will suggest frequencies per country. See what products have appropriate crossover.
If you have a significant amount of devices, you may wish to download Sennheiser's intermodulation software from the site and insure your system components will not interfere with each other.
I believe in the States available frequencies vary from state to state, I know Shure have a very comprehensive guide on their USA site, a friend of mine used it when he was in America recording sound using wireless Lav's for a documentary.
In the US, most wireless microphone/IEM applications do not require a license. It is the user's responsibility to stay outside licensed FCC bands in every city, however.
If you have frequency agile systems that work within the US-specified RF bands, it should not be a problem, but you have to work outside cellphone, military, DTV, analog TV, radio broadcast, and similar bands.
>>did you know that a foreigner IS NOT ALLOWED to get a licence for using his own wireless system?
so if I go to the US, buy my favourite Sennheiser-System, I am not allowed to operate it. but.. if I rent it, I am allowed (because I am not the owner).<<
George, this is terrible news. Where did you information come from?