The Unique Visitors referenced within the App Analytics for your event will indicate the number of unique devices that have accessed your event app. While reporting on the number of users who have logged in is also available, this metric will provide you with better insight into how your attendees have interacted with your event app. For example, a user who has accessed the app from their desktop prior to the event, and also from their smartphone during the event will be captured as two instances of a unique visitor.
It is highly possible that the unique visitors' number may exceed the actual number of attendees for your event, especially if you have used registration, or self-edit emails to help you in promoting the event app and directing people to it.
The Platform Breakdown area of the App Analytics page will provide additional information regarding the device types and platforms that were used to access the app.