I think this is 'perhaps' what you’re asking for...

In the Sonos App, goto the ‘Rooms Tab’ on main bottom bar and 'select’ either the Play 5, or the Port ‘Room’ in the list and choose the “group” button and check/select the boxes, so that both devices/rooms then group together. Click ‘Done’ to return to the Tab. 

Now goto the 'Browse Tab’ on the main bottom  bar ..and select/stream your chosen audio source to both rooms in perfect sync.

Note: you can also ‘long press’ on the Sonos volume-slider control in the App to reveal/control the volume of both rooms in the selected group.

Hi User573051

Not understanding what you mean by “Force Group”. I assume you want music started on your Play 5 to be heard through speakers connected to an AVR.  Here are the steps:

1. The Port (named as a room) should be connected RCA-Out to an RCA-In of the AVR.
2. Select whatever source the RCA’s correspond to on the AVR such as Tape-in, CD or AUX.
3. Start the music on the Play 5
4. Group the Play 5 to the Port 

Edit: Looks like  @Ken_Griffiths  has already answered  your question while I was typing 

The only other reason for your question would be that you want the Play 5 to start playing music when you select a TT or CD connect to the AVR. I that case you would need a Tape-Out on the AVR going to RCA-In of the Port. You designate the Play 5 as the auto-play room.

If this is the case and there are no speakers connected to the AVR. You could connect the CD or TT with pre-amp (built-in or outboard) directly to the Port.

I do this and it works but when I turn off my system, it goes back to the Sonos Port not being group and I have to group it again.

What do you mean by turn off? Are you unplugging the system from power?

Sorry: when I just stop the music either through the app or the play/stop button on the Play:5.

Thanks. Grouping shouldn’t break when just stopping, unless you have un-group on auto play ticked. It wouldn’t survive a power cycle, but should survive a simple ‘stop’

So, I should uncheck “autoplay” ?

No, autoplay would be fine. There should be another line that says ‘ungroup on auto play’ or words to that effect. 

Like this ?

If you click on autoplay room, what does that screen show?

User573051,

I suspect you are perhaps having this issue, because you appear to have two Sonos “Rooms” with the exact same name of “Living Room” showing in your screenshot above … what are each of those two identically named rooms?

Can you perhaps rename one of them in “Settings/System/eLiving Room]” so the system can tell them apart from each other and see if that resolves the matter?

Also can you clarify if you use Amazon Alexa with your Sonos setup, as that too can have an effect on auto-grouping/ungrouping your Sonos rooms through a thing called “Amazon Alexa Groups”.

In fact, you are pointing to something that was unclear to me about rooms: I have my Port and my Play:5 in the same room (that’s why I want to force group them) but it looks like room = one device only. Is this correct?

I do not use Alexa.

Yes, give the Port and Play 5 different room names … that should hopefully solve your issue and stop them ungrouping.

As Ken is suggesting, names in Sonos are speaker identification, not physical location identifiers. You could easily call the two rooms Peter and Paul. I’m not sure how the Sonos software works exactly, but changing the room names would be a good test. 

In my home, some ‘room’ names are the room, but in some cases, I’ve just used the type of player. I know that the room named PLAY:5 is my guest room, or Sonos One is my bathroom. That also helps me identify what speakers I’m using when I use AirPlay 2. 

Works perfectly now. Thanks.

Interesting. I’ve always assumed Sonos ignores names, and uses speaker IDs in the code.