Whilst you can connect the Beam to provide the front three channels via and HDMI connector, Moves cannot be used for surround duties. 
 

Theres lots of information on Sonos’ web site, so have a look there and come back with any more questions. 

You’d need to connect the Beam via HDMI-ARC, which your Mac doesn’t normally generate. The ARC signal is usually created by the TV, and sent out via the port labeled ARC (or eARC) to the connected speaker.

You could create a Dolby Digital signal from the Mac via HDMI to fairly common HDMI switch that has an optical port on it, and then connect the Beam to the switch using the Sonos adapter to an optical signal from the switch.or, you could use an ARC generating device between the Mac and the Beam. 

There is the HD Fury Arcana, the Feintech VAX04101k, and the OREI HDA-935. You can also order from those links from the companies directly, or check your local retailers for availability. Amazon may carry some, depending on location. At this point, there may be more manufacturers, I haven’t been keeping track.

In general, this isn’t easy, due to the type of signal fenerated by the Mac.a lot of this is likely covered in this area:

https://en.community.sonos.com/groups/sonos-products-as-pc-speakers-229137
 

Edited to add: and ​@nik9669a point about not using Sonos Moves as surround speakers is extremely relevant .

Thanks guys - so if I restate the question -

I’d like to have the iMac sound output redirected and in sync to 5 speakers (beam, move x 2 and roam x 2) and not worry too much about surround-sound channels, is this doable? Bluetooth seems not to be a solution given the lag.

Many thanks again

Since you want to watch films, the short answer is “probably not”…
 

When you group the other speakers with the Beam, there will be a mis-match of audio to those other speakers. Those will all be in sync with each other, but playing approx 40msec delayed from the Beam’s output. This is inherent in the way Sonos works. It’s when, for your setup, the source is via HDMI-ARC. The Beam will appear to be sound in sync with the on-screen video, but those other speakers will muddy that sound. Stick to just the Beam and you should be fine.