Top influencers are group members whose contributions in discussions (threads started, comments, and likes) drive the most participation from other members over the course of a week. Members with more followers in the group are more likely to be influential in driving participation among others.
LinkedIn keeps track of this participation and shows the results in the Top Influencers This Week box, found on the right of your group page. Clicking on a profile image or name will take you to that member's activity page.