You’d need to use the Sonos controller to ‘remove’ it as a surround speaker for that home theater room, and set it up as a separate room (note, this will cause loss of any TruePlay settings, if you have them, as the room’s speakers have changed). You then have the Era 100 as a separate ‘room’ in the software, and you can move it anywhere you want. 

I could be reading this wrong, but I think OP wants to add the Era 100 as a surround speaker, but remove it.

First off, you need to a matching pair of speakers for surround sound.  So two Era 100s, not one. Each your speakers will first be setup as their own room.  If you have a stereo pair, go to the settings for the room and select the option to remove a speaker, which will automatically setup as another room (not 100% sure this is necessary).  Then go to the settings for the room with the Beam 2 and select the option to add surround speakers.  The wizard should let you select the two speakers you want to use as surrounds and any other configuration needed.

Right, Danny. Re-read my post, which is about removing the Era 100 as a surround speaker.

Or, have another cup of coffee ;)

Hi again

Thank you for your feedbacks

Today I have an Era 100 in my office.
In the family room I have a Beam 2 + Mini Sub.

Now I want to remove the office and the Era100 speaker and use it as a surround speaker in the family room together with an extra era 100 that I have bought.

How to remove the Era 100 from the “office” and add it to the “family room” 

Try here

https://support.sonos.com/en-us/article/add-surround-speakers-to-a-sonos-home-theater

Aha, I misunderstood your phrase: Must now be used as a surround speaker together with BEAM 2 and Mini Sub , which I thought was its current state.

Moving speakers is not as easy as one would like between rooms. Particularly when adding/removing as a surround speaker. There are likely several reasons for this situation, but following the instructions as provided by @melvimbe and @Stanley_4 will get you where you need to be.

Edit: Superfluous 

Always loved that word, it brings the question to mind: what is regular old ‘fluos’ like, before it becomes superfluous? I suppose I should look up the etymology at some point, but I like the smile it brings ;)