Other than a second Sub, you have maxed out your system. You cannot add any more front or surround speakers to your current setup.

@dannielmo,
If you do, you will only be able to ‘group’ (not bond) the front Era-100s to your HT setup. It may ‘muddy’ the front channel TV audio. See below…

Note: Any Sonos ‘grouped’ rooms connected to a Sonos HT room will play ‘music’ audio in sync, such as Sonos Radio and the other music sources that you see in the Sonos App. However any ‘TV audio’ will only play in stereo on the grouped room(s) and will be out of sync/lip-sync by ~75ms minimum with the bonded HT speakers and may cause a noticeable echo if placed in the same TV viewing room - it’s better practice to have the grouped speakers therefore located in other physical rooms in the Home. 

However there is a feature in the Sonos App HT room settings called ‘TV Dialog Sync’ that can buffer the Home Theatre audio on the bonded HT speakers to ‘perhaps’ bring the TV audio into sync with grouped rooms, but this in turn may cause lip-sync issues with any video seen on screen.

Also, all channels will be mixed into the grouped "front" speakers; front center, surrounds and heights.  This will mess up any surround or height directionally and further muddy the front soundstage.  In short, don't do it.