I have a single pole circuit and was wondering if the add-on switches must be wired to the master switch or just have power? I have a multi-gang box in the bathroom and could install the add-on switch in this box but it is not wired to the bedroom light (a single pole circuit.)
If you’re using the common GE/Jasco/Honeywell, the add-on switches don’t work independently. They communicate with the master switch over the wired connection.
If you want control from a second location, you could put another smart (master) switch in this location without a load and connect it programmatically, or use another button / scene controller.
(edited for clarity per JD’s comments below)
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It depends on the exact brand/model of the switches that you are using. The GE, Inovelli, and Leviton three-way set up do use physical traveler wires. But many of the other brands including Linear/GoControl, Eaton Cooper, Homeseer, and Lutron Use “virtual” three-way setups where both the master and the auxiliary switch have a radio. The auxiliary sends a message to the hub and the hub sends a message to the master. No physical wires are required between them, they can even be on completely opposite sides of the house. There are pros and cons to both approaches, so it just depends on what you need for each specific switch location. 
Example of a virtual three-way add on switch.
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