Yes you can use the ARC as an input for your turntable. The turntable sound can then be played on any speaker, you don’t need to group it, the Turntable will show as a Music source for the arc in the Sonos app. You can also set the auto play room to be the room the Arc is in. That way every time you play a record it will automatically play on the arc.
From a a home theatre aspect you cannot use a single Five, or any single sonos speaker, as surround, they have to be a pair,. I have a pair of Ikea Symfonisk speakers as surrounds, great value.
You could also consider a Port as the turntable input and get the two symfonisks for not much more than a Five.
For using ARC as input, I’d need a Port right? Same for the symfonisks. I understand you connect the turntable to Port and from there you connect it to the Sonos ecosystem (something like that)
Based on the product page of the Pro-ject T1 product page...
Compatible with Five, Play:5, Amp, Port, Connect y Connect:Amp
Since my TV + Arc would be in the very same room as the turntable and the speaker, I’d love to use them all together. Both for TV or music.
It actually makes sense to use a pair as surround 
Think of the port as a Five without the speakers!
If you listen to a lot of Music in that room then get two Fives, that’s what I would do, you can connect either Five to the turntable. Then no need for the port etc.
As for the turntable make sure you get the version with an integrated Phono amp, or buy your own as an extra. Up to you.
Thanks!