Adding New Speakers creates a new system

Something is not clear in your post. A “new” system would require you to set up new log-in credentials. Could it be that you can only see the Era 300’s when your device is range of an extender or network node if you have a mesh setup? If so that means those devices are generating IP Addresses independently of your main router.

Also wiring the Arc has established the SonosNet which the Era 300’s cannot join. Try unplugging the Arc and the Era 300’s and then remove the Ethernet from the Arc. Reboot your router Plug in the Arc to make it join your home WiFi. Next plugin your Era 300’s as they should rejoin your WiFi. Let us know if all speakers can be seen just by opening the App.

Do you really create a new system, or just a new “room” in the app. A screenshot would probably help in determining this.

It's a new system - I have to switch systems in the app to see the added Era 100s. I am aware of the difference between rooms and systems. I can also have multiple rooms in the second new system that gets created when adding the Eras

When I am adding the Eras, I've done this with the "Add Speaker or Component" button in my primary Home System. I would expect a new room at most.

​@AjTrek1 mentions about attempting unplugging the Arc, and joining the Eras when everything is wifi - could the new system be happening because the Arc is wired in?

When you add the new speakers to your WIFI, at first they will be seperate rooms. To add them to the Arc, you will need to tap the gear icon in the top right of the screen, then select the room with the Arc. Scroll down and you will see Setup surrounds and follow the instructions;

https://support.sonos.com/en-us/article/add-surround-speakers-to-a-sonos-home-theater

Alecks wrote:

106rallye wrote:

Do you really create a new system, or just a new “room” in the app. A screenshot would probably help in determining this.

It's a new system - I have to switch systems in the app to see the added Era 100s. I am aware of the difference between rooms and systems. I can also have multiple rooms in the second new system that gets created when adding the Eras

When I am adding the Eras, I've done this with the "Add Speaker or Component" button in my primary Home System. I would expect a new room at most.

​@AjTrek1 mentions about attempting unplugging the Arc, and joining the Eras when everything is wifi - could the new system be happening because the Arc is wired in?

Did you find a solution? I have the same problem. I have factory reset the entire set of speakers at my place trying to get them all onto 1 'system" and my Beam 2 keeps creating its own system. 

My system consists of:

2 x Play:1

2 x Sonos 5 gen 2

1 Beam gen 2

1 Play:3 

1 Sub mini 

It was fine up until a couple months ago, but then the TV room started showing on its own system (System 2) apart from the Living Room and Bedroom. (System 1)

This is how I have my HT system setup and I can confirm it does not cause issues when adding new speakers to a system. 
 

Have you tried resetting the new speakers, then uninstalling and reinstalling the app on your phone? I would also recommend restarting your phone. Then reconnect to your existing system and add the speakers again, making sure you are adding to existing system and not creating a new system.