To add a turntable you need a Sonos device with a Line-In; a Five, Port, Amp or the new Era 100/300 with the extra Line-In adapter.  All of these allow you to choose the Line-In source on any and all other Sonos devices in the system.  You can also group all devices to play the same source in sync. 

So if I understand correctly, I can add the era 300 to my AMP and Beam gen 1 will still Work?

It depends on what you mean by “work”.  What exactly are your expectations?  Do you want to play music together?  Do you wish to have surround sound on the Beam?  Do you just want to listen to your turntable over the Beam? 

Good question!  Currently the beam is my sound for tv, streaming music and shortly turntable. I’m thinking of keeping beam as front speakers for everything and add wireless eras or something similar for surround sound. Possibly a subwoofer if needed…. Thank you!

Today, the cheapest way to get a turntable to play on your Beam is the Port.  In the future it will be an Era 100 with the adapter.  Note: If you do get Era 100s as surrounds, the Line-In is disabled in surround mode.  You would need a third Era 100 for the turntable. 

Thank you for your information.  I am planning on adding the Era 300 (with adaptor) for my turntable only.  I am hoping to expand, if I feel I need to, at a later date.  The reviews I have read about the Era 300 are very encouraging.  I really like your product and the ease of use and that’s the reason I have decided to stay with SONOS.  Thank you again for your assistance.

Seems like a sound plan...but just to note, @jgatie is just another user like you and I, and not an employee of Sonos. Those folks are required to have accounts that are marked as such. However, @jgatie is indeed one of the most knowledgable users here on these forums. :)

Is their a Sonos line-in adapter alternativ ? 
It’s not in stock for a another month or two or should I just wait?

I read you could use a different adapter, but need to make certain it works from the audio in to the USB out - not all adapters do this, see https://nl.community.sonos.com/speakers-229124/lijningang-era-100-werkt-niet-6775810

The Dutch Sonos employee states it like this (google translation):

“It is not the case that you can only use the Sonos adapter specifically. What you have to take into account is that the Sonos adapter works in reverse. Usually all adapters work from Device to audio jack (usually headphone) Sonos uses the adapter the other way around Audiojack to USB-C. Not all adapters support this.”