A number of sites and apps is available for playing geocaching.
We use geocaching.com. It is compulsory to register an account for your team, either a regular FREE account or a paid account. Premium accounts offer more tools, but for the concours it is NOT necessary to have a paid account.
For Android we advice the following free app: c:geo which can be downloaded from the Play Store. For iPhone Geocaching.com provides a free app which can be downloaded from the Apple App Store.