I don't believe there is a way to get a list of all currently connected users. You can however track the connection attempts as well as successes/failures to the AF server over time via WCF logging as described in KB00412. You could use this information to backtrack to determining the number of unique users connecting to the AF Server or the total number of connections being made to the server in a day.
What is the larger context here? Do you want to know how many people connect for performance / load reasons, security concerns, etc.?
If you simply want to monitor who connected, then Robert Schmitz response should get you going.
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In the upcoming PI AF 2018 release, the ability to get the session information for clients that are connected to the AF Server has been added. You will need an updated AF Client and Server to utilize this new feature. It will be accessible from PI System Explorer or programmatically using the PISystem.GetSessions method.