When you have agents with multiple fields of expertise, it is natural to expect a functionality where you could add them to different groups. Please navigate to Admin -> Team -> Groups -> click on edit so that you would take to the page with the configuration options where the agents could be added. 

Another alternative would be to go to the agents' profile by navigating to Admin -> Team -> Agents -> click on edit next to their name and add the ones the agent should be part of under "Groups."