Identifying an activity specifying thematic content for a video exchange session between users of an online system using a trained model

When a user creates a new video exchange session via an online system, the user specifies an activity for the video exchange session. The activity may specify a name for the video exchange session, as well as other thematic content (e.g., visual effects, backgrounds) displayed via the video exchange session. The online system trains a machine learned model that predicts a likelihood that other users will accept an invitation to join the video exchange session if a certain activity is specified for the video exchange session. The online system may then use these likelihoods to rank and sort the activities for the user or to highlight activities with threshold likelihoods.