Interests groups both advance and hinder democratic values, the answer lies in your own perspective. They advance values because they serve as an example of "By the people, for the people" since they represent the interests of a group of citizens. They further help their voices be heard in many venues, mainly political ones.
On the other hand they could also hinder values because interest groups or lobbyist that represent private citizens and entities can blur the lines. The interests of these entities and people can be heard more because they may make donations to political figures, making it easier to have their private interests outweigh that of a normal citizen or interest groups that represent them.