You can add up to two Subs (one must be a Gen 3) and two surround speakers to a Sonos home theater setup. So if you want to use the Era 300s as surrounds, you have to replace the Era 100s.

I was thinking of doing the same thing to add 300s to existing Era100s but I guess it won't work as per Sonos.  I have added one 5 to my system and seems to work OK but a slight delay ( echo) with the sound but with music from Sirius radio sounds great, so if you group extra speakers you will have a slight delay. If your going to watch TV it may sound a bit off. (Echo) but it's up to what you like and if you can watch TV with that sound. Everyone has their own sound preferences.  Just my opinion.  Good luck.

So grouping two Sonos rooms together is entirely different than bonding speakers together in a home theatre Sonos room.  With bonding, each speaker is playing a different role, different audio channels effectively (atmos is a bit different).  The speakers are designed to work together.    Temporarily grouping a room with a Five in to your home theatre room does not have the Five playing an assigned role in the home theatre.  The Five is just playing a stereo/mono copy of the audio.

As far as the delay goes, speakers in your home theatre room are connected via 5 Ghz WiFi, which has limited range but faster than 2.4 Ghz.  This allows the speakers to play in near perferct sync with the video.  The grouped room is communicating over 2.4 Ghz and buffered for multiroom audio.  This also speakers in different rooms of your house to play the same audio.    When you are playing music streaming sources, everything is slightly delay so that everything is in sync.  No echos, other than speed of sound.  For TV sources though, you will hear an echo since the home theatre room plays early to match the video, and your other grouped rooms do not.

You are correct that everyone have their preferences, but I wouldn’t want to misleading one into thinking you can just add speakers as you wish to a home theatre room  if they want extra front speakers or extra side speakers.  That’s not what happens when you group Sonos rooms together.

Thanks for that reply and I get it but was thinking if returning my 5 for the reason you described, but was thinking of the 300s to enhance the sound much more than the Era 100s. So as described, if I group these 100s with 300s I will still get an echo? To be honest I didn't mind a bit of echo and only bothered my wife lol so I think I may just leave it as it is. Thanks again 

Grouping with the TV sound always gets you the delay on the speakers grouped with the TV’s Room.

There is no delay or echo with grouped speakers playing streaming (non-TV) audio.

What is playing when on speakers playing from the same streaming source individually (not grouped) can vary widely depending on the source.

Just to be clear: a Group of speakers playing music will be synchronized with each other. Audio associated with a TV playing in a surround room will be delayed as it is sent to a Group. This will cause issues if TV audio is played by a Group member located in the same physical space as the surround system. There may not be any noticeable issue if the Grouped speakers are in distant rooms.

There is a SONOS TV Dialog Adjustment that might help synchronize Group and TV Voice sync at the expense of some Lip Sync. Each situation is unique in this respect, your mileage will vary. Note that there may be some sync adjustments in your video sources and TV that may be productive or problematic as you fuss with sync.

OK another question.  I  want to add a gen3 sub to my system. ARK, mini sub and a set of 300s.  Now if I grouped a set of 100s and added a gen 3 is it possible? I  won't be running two subs at the same time, just in a different group. 

The stereo paired 100’s and their SUB will be a unique, separate Room that can be Grouped with other SONOS Rooms.

Thanks for that answer. So what I am going to do is purchase a gen 3 and connect it to go with my Ark and 300s,  the group the mini with my ser of 100s.  One set for listening to TV and the grouped set of 100s with the mini for music. This sound would be perfect for me. Thanks again and out to bestbuy tomorrow. 

Look for the Sonos Arc, not the Ark…;)

I have a Sonos Ark plus a set of 300s for my surround plus a mini Subwoofer.  What I want to do is purchase a sonos gen three and add that to a set of 100s.  So my 100s are in a different group where I play music. I want to keep the mini but want to use the gen 3 sub for music not running both subs. Is this possible when they are set in another group.  

Sonos Arc. And the 100’s will be in a different “room” not “group”. You can group different rooms, but not use different devices out of one room in another.

I appreciate the detail in these discussions, especially about bonding the home theater versus grouping in the room, ..I have a question about trying to do both at some time?  To incorporate the arc, two fives, a sub, and an Era 100 ain adjacent room.  I use Alexa through the arc, but would like to play all the speakers at the same time.

Hoping it’s possible as I have these really nice speakers and would like to connect them all together as main floor speakers if possible well, keeping theater set up with the arc two fives and sub.

The surround system is a “Room” in SONOS parlance, soundbar, surrounds, and SUB become part of a surround Room. Two identical speakers, such as FIVE, MOVE, ROAM, etc., can be “Bonded” as a stereo pair and become a Room. Rooms can be “Grouped” to play the same music. While playing the same music, all members of the Group will be time aligned. If you are feeding a Line-In from a CD player, turntable or such, there will be a 75ms time offset between the Line-In and output from a SONOS player. This time offset will fluster most attempts to use SONOS as a home DJ system. TV sound is a special case because a 75ms latency too long for effective synchronization between video and audio. Therefore, the latency is reduced to 30ms for soundbar input from the TV. If you Group other Rooms with the TV audio, since they are at 75ms latency, attempts to play a Grouped speaker along with the TV audio will be frustrating. If the Grouped Rooms playing TV audio are in a distant room, such that only one Room can be heard, the 75ms offset is not important. This latency difference will frustrate your attempt to play your ERA 100’s along with the surround sound.

Why is there latency? There are frequent network stalls that can be caused by other traffic on the network or radio interference. A one millisecond delay while downloading a web page is inconsequential,  but it is a disaster for audio. The latency allows SONOS players to ride through minor network stalls without any consequences for the listener.

Again, while playing music all of the Grouped speakers will be time aligned.