From a group page: simply click on 'Add' to open the search and instantly add any contact from your folk network directly in your chosen group.

From the 'Global search': this search is designed for your productivity, making sorting and categorizing contacts as you go as easy as possible. After finding the contact in the search, you can either click '+' and it will add the contact in the context of the group displayed behind, or open the contact panel to add the contact to a chosen group.