There is no search functionality for CB Activity. Most of the content is dynamically displayed based off the value of the "asset". The exception being user posts. Filtering will eventually be available in a later release. Tagging is supported using #HASHTAG though, which can filter posts down to the used hashtags. For CB GroupJive searching that's already built into the various GJ pages and/or using GJ Search Bot and Joomla search module but it will only search group name and description.