The 100s can only be used for surrounds and should be behind and to the sides of the main viewing area.  The Ray is a soundbar, and already includes the front left, center, and right speakers.

I put them on the sides and it seems to sound better. The sound is much more spacious. The only problem is, I can't figure out if Sonos recognizes them as front speakers or rear speakers.

A little later I want to change the configuration by adding 2 more Era 100 speakers, but already in the background. But the question is, is there any point in having 2 Era 100 speakers in the front with a Ray soundbar?

Right now in the Sonos app, the speakers are defined like this.

RS and LS stand for Right Surround, and Left Surround. As @jgatie says, they should be behind you, in the ‘surround’ position.

There is no way to make any Sonos soundbar a ‘center’ speaker, and flank it with right and left channels. 

Do I understand correctly that there is no point in buying 2 more Era 100 speakers for the rear? It's very strange that Sonos doesn't allow you to augment the left and right front channels. I don't think even the Arc soundbar is capable of fully processing the sound for the front left and right channels.

Ideally Sonos would allow you to put a center soundbar, 2 front speakers and 2 rear speakers. Too bad it can't be done that way :(

Kind of. What you should be thinking is there's no reason for a pair of Sonos speakers flanking your Ray, those extra pair of speakers should be behind you. 
 

I have two Arcs, both contain right, center, and left channels in disparate speakers, there is absolutely no difficulty in them processing the sound for the right and left channels. Much as I expect the same in the smaller ‘box’ of the Ray. Just less distance between those disparate channels. 

Soundbars, SONOS or otherwise, are intended to be convenient to setup with a minimum of wires. Traditional surround with many more speakers, wires, and much more complicated setup, offers a more enveloping environment. It’s your choice.

The Era 100’s you put in the front are surrounds, not front speakers, and are meant to be behind you and to the sides.  Surrounds are for two things, directional cues like a bullet whizzing by or a plane going overhead, and ambient sounds like rain or traffic .  With the surrounds up front, all the sounds meant to travel by you to the rear of the room are all up front, so there are no directional cues at all, and the ambient sounds are mixed in with the real front sounds in the Ray, muddying those up.  In all honesty, it couldn’t sound any worse.  Put them behind you, you will thank us later. 

Possibly because of a fractional delay which Sonos may introduce (I don’t know for sure) to time-align surrounds that are usually closer to the listener than the soundbar.

I returned the Ray to the store and bought an Arc. Era 100 on the sides in the back. I put the surround sound on. But the sound in the back is not really audible, it's there, but I wish the sound was really 360. Maybe I need to change something in the settings?

I've temporarily placed the Era 100 on a table. I'm waiting for the stands to arrive.

In the Sonos app settings, I found two settings for surround sound (TV volume and music volume). But I still don't know what they do.

When the stands arrive I plan to place the Era 100 as shown in the photo (red color). But again it is not clear where they should be directed.

I was planning on placing them like this. But I'm not 100% sure if this is the correct placement.