If you are still using the S1 app (old version), the Era 100s will not work because they are only compatible with the S2 app (new version). See here:

https://support.sonos.com/en-us/article/sonos-app-version-compatibility

Depending on if your older speakers are compatible with the S2 app, you can upgrade them to the new app and add the Era 100s to your system then.

If your speakers are not compatible with the new app, like the Play 5 Gen 1 for instance, you then have two choices: download the new app and use the Era 100s separately, or return them if you do not want two separate systems.

If I don’t want two separate systems, does Sonos make new speakers that are compatible with the first Sonos app?

No.  All new speakers are S2 only. 

Thank you both for your time and kindness for helping me. (I am an older person and this is all a little over my intelligence level). Thank you!

What existing kit do you have, ​@D Cast? You may be able to run everything on the S2 app. 

I have a Play one and a Playbar 

I will try the S2 app. Thank you for your advice. 

I’ve certainly run S2 on both a PLAYBAR and a PLAY:1 with great success. 

This guide might be helpful:

https://support.sonos.com/en-us/article/update-to-the-latest-sonos-app-using-the-s1-controller

If you are still uiting a Sonos Bridge go over to WiFi or a cable connected speaker first, since the Bridge isn’t S2 capable. See https://support.sonos.com/en-us/article/remove-a-bridge-from-your-sonos-s1-system

Thank you for your help.