Rivet Networks has joined my Wifi alert

For the first time, I've received an alert from Xfinity "Generic Brand Network: Rivet Networks has joined your network for the first time".

a) There are no new people or devices in the apartment at the moment.

b) I don't know what this is.

c) How is it able to join the network?

Note:  I have an existing, unresolved posting ("Why do I keep getting these alerts") about continuously getting alerts that a device has joined for the first time when the device(s) in question have joined many, many times.

Thank you for any assistance.