From the LinkedIn Blog:
Group search helps you search across all the groups on LinkedIn and find the right group for you. We’ve made several improvements to make your search results smarter and more relevant.
First, instead of relying on the title and description of the group, we give you the best results based on how well your search matches the conversations taking place. We also show your connections who may be in that group, which makes it easier for you to find groups that really matter to you.
Filtering your results is improved as well. You can now filter your results by your network, categories, and also language.
Direct to LinkedIn Group Search