Group Complete by Radical Dynamic was an awesome ODK-compatible data collection and aggregation platform for Android devices. It was known for features like seamlessly syncing forms across devices, pre-populating forms with data from Excel files, and flexible user permissions on groups of forms. You could even design forms on one device and push them to other users' devices! Unfortunately, due to a change in Radical's direction, the service was shut down in February 2012.
Today, Matt Adams, one of the Radical's founders, has graciously open-sourced the Android client for Group Complete. The code repository is fantastic place to start if you are looking to build group-oriented and ODK-compatible data collection.
Matt is a fantastic member of the ODK community and we are deeply appreciative of this contribution. Again, the repo is at http://github.com/radicaldynamic/gcmobile. If you intend on releasing derivative works, please pay close attention to the README.