The Boost sets up dedicated network called Sonosnet, to which you cannot connect the Era’s. However, when they are used as surrounds the Era’s do (like all Sonos surrounds and Subs) not connect to Sonosnet but use a dedicated 5Ghz link to the soundbar. So it is irrelevant if your speakers are connected through Sonosnet or not. You can just ad the surrounds to your Arc or Beam (Gen 2) as you would do other Sonos surrounds.

I originally tried this option but the message in the App stated the Era were not on the same system.

It appears you set them up as a separate system.  You need them on the same system.  Factory reset the Era 100s, then start up your actual system in the app, and choose to Add a new product to an existing system.

So are all your devices showing in the Sonos App? Check the ‘Settings/System/About My System’ area if the App. Ensure the Deco WiFi bands (2.4ghz and 5Ghz) are not isolated from the other network segments (AP isolation)

If no joy with AP isolation, perhaps see what happens if you perhaps run all your Sonos devices on your Deco WiFi instead of SonosNet? See these links to assist with that…

https://support.sonos.com/en-us/article/choose-between-a-wireless-and-wired-sonos-setup

https://support.sonos.com/en-us/article/switch-sonos-between-a-wireless-and-wired-setup

jgatie, If I add them as a separate system, can I add them as surrounds to my Beam and Sub that are on the Boost side of the network?

You need to add them to the same system as the others.  Sonosnet and WiFi speakers can coexist just fine on the same system.  I’ve had a Move and a Roam on a Sonosnet system for years, and they are WiFi only.  Note: I clarified my murky instructions above.