member? can be used with lists or agentsets, but is more commonly used with lists. member? reports true if the given value or agent appears in the given list or agentset. For example, member? 3 [12 -5 3 6 1] would report true, while member? 5 [12 -5 3 6 1] would report false.

In the model example below, we have a building that represents a library and some turtles who represent patrons. While the turtles move around randomly, our library checks at each tick if there is any patron nearby. If there is a patron, our library checks if the patron is a member of the library. If the patron is a member, our library asks them to change their shape to indicate that this patron either picked or dropped a book.