It depends.  If the turntable has bluetooth you can connect via that.  Although that will kill the sound quality.

Otherwise the Turntable will NOT connect to the ARC via a line in method.  The best solution is get a Port or a Sonos Era 100 with the Sonos Line In Input adapter(Cheaper than a port).  Then you will need a Phono amp for your turntable, unless it has one built in, and connect to the Era or Port.  Sound from the turntable can them be share to any/all of your Sonos speakers.

You have a three (3) options (the latter you didn’t use 😊):

1. Connect it to your Play Five via its line-in using RCA to 3.5mm cable and then group-in the Arc.
2. Buy a Sonos Port and use its line-in option and name the Sonos Arc as the “Auto-Play Room” (no grouping of another speaker required). *
3. You could have gone to the Sonos Home Page, clicked on Support and typed Turntable in the search field and you would have been taken to the page shown in this link 

As you are buying a Pro-Ject turntable through Sonos all have a Built-in Pre-Amp. If you weren’t buying through Sonos you’d need to make sure that any turntable you purchase has a built-in pre-amp or purchase one to sit between it and the Sonos product with line-in capabilities. 

If you want a complete wireless experience without cables you might consider a Victrola Turntable shown art this link. 

 * The Port will allow you to name any Sonos speaker as the Auto-Play room and you can still group-in other Sonos speakers.