Currently a team of developers is working on Android application and during the development process testers already have to test the current state and report issues.
So
There is also software called Installapk that allows you to install APKs on your device very easily, though it is only available for Microsoft Windows, and is currently in beta.
Installapk
Here is what I do for that, simple: Build in debug mode, no need to sign with a special key. Mail them the application by regular email to their computers. Then have them connect their USB cables (comes with the phone) and copy to sd card. Access the sd card from the within the phone using any available file browser, and they can install it from the SD card. Regards
You can allow untrusted applications in the settings.
Settings / Applications / Unknown sources
with that setting on, you can just point your phone to the url of an .apk and install it IIRC.
It is also possible to install apks using debug mode and adb-commands over USB-cable.