Ramshare is by invitation only, so you need to know someone, or know someone who knows someone. Mostly because this is very experimental and needs more testing and more features before it goes out to the public.
Right now, there is only one supported emulator, which is a modified version of the popular gambatte-speedrun gameboy emulator. A windows version can be downloaded from github. For the more adventurous, try building it from source. My plan in the future is to transition to mgba and only requiring a lua script to be loaded. As of the start of 2020, lua scripting has not been implemented yet.
In the future, other emulators might need to be tweaked, in order for ramshare to work but the ones with inbuilt scripting are likely going to be developed first.
Ram share does not host roms. However the internet is a large place. Currently only two games are supported, that is Pokemon Red/Blue and Pokemon Crystal. It is safe to pass any roms through the universal pokemon randomizer and have everything be okay. For pokemon Red/Blue, it seems safe to use stump's key item randomizer. With his blessing, it might even be hosted on this site eventually. For those worried, pokecrystal-speedchoice is also compatible with regular crystal, so that's cool.
After getting the emulator, and rom. It's time to start playing? Not quite yet. Firstly, you need to go to your profile page and download your client_auth.json file. This file is basically the configuration for the emulator, to communicate that it is you actually sending the game data. Every time you download your client_auth.json, it will generate a new authentication key. This so if yours ever gets out, it is very easy to reinitialize it. Doing this will invalidate the old key. Put your client_auth.json into the same folder as the directory. If you're lazy like me, you just leave everything in your downloads folder. After that, start the emulator and you should be cooking! Important: you will want to rebind the "ramshare" button in the input settings. Ramshare will be updated everytime that button is pushed.
There is no intelligent way to determine which games make sense for which game you are playing, so a drop down list is what you get. After choosing your game, give the room a classy name, and add the other players in a comma separated list. Note, this must be the player's usernames (what they used to log in with). Add new!
Configure the game how you want to and click "Update Rules/Save Game" in the "Room Controls" options. Then click activate!. Emulators trying to communicate with an inactive room will not update anything. It will also start the timer. There is a reset game option, it is strongly recommended that the emulators are fully closed down before trying to start again, as something there might be residual data in the ramshare buffer in the emulator.