I've been working with GroveStreams for a while now and I wanted to share some of my work. Here is a blog post with step-by-step instructions to build an XBee-based wireless sensor network that sends data to the GroveStreams platform. It builds on a couple Arduino libraries that I published on GitHub. These are linked from the blog and all necessary microcontroller code is included in the example sketches.
There is also an example sketch for a standalone GroveStreams client (i.e. no wireless network) that requires only an Arduino Uno, Ethernet shield and a sensor to generate the data to be logged.