You can group the Era 300 with the Arc setup, but you cannot add just one speaker as a surround. It has to be a pair of speakers. Also, when you group the 300 with the Arc, you will experience a slight delay from the 300 when playing TV audio on the Arc.

Hello! How do I group one Era 300 to Arc and Sub setup? 

If you mean group together to play music in sync, see this link:

https://support.sonos.com/en-us/article/group-and-ungroup-rooms

However, that isn’t surround sound, and it isn’t optimum for TV sources because the grouped room will be delayed.  For surround sound, you need two speakers to bond as the surround Left and Right. 

Thank you very much for your reply! Very helpful!

I also suggest buying a second Era300 and to set up all speakers as a Atmos surround setup. 
If you can’t afford buying a second Era300 right now and like to use one Era300 and Arc Setup in a group for tv sound, the delay on Era300 that @jgatie mentioned can be eliminated by using the option „tv dialog sync“ for the Arc. Setting the leverl there to 3 should minimize the delay so that both rooms sound synced. But be aware that this option creates a delay of Arc Setup to tv picture. For me with level up to 3 the lip sync delay is not noticeable. But more would result in a lip sync problem.