Mrs. Jennings went out in the mornings and left cards at the homes of her friends. Willoughby also left his card while Marianne and Elinor were out with Mrs. Jennings one morning.

I found this interesting L&T post on calling cards, linked below (it also links to other interesting posts on the same topic).

This apparently was something that would particularly be done when the families arrived in Town, so you would know who had come. Not leaving a card and letting people know you had arrived was a social faux pas, causing some people to stop seeing each other if one had neglected to leave the other a card!