What is it?
The GUID that the Arma franchise uses is merely another name for the steamID being converted to the so called 'steamID64' or as we use in RO2, the famous 'ROID'. This being converted from your steamID/customURL you use means that this ID is unique for every single user on steam, hence why it is so widely used in the Arma franchise.
Creating the actual GUID/SteamID64/ROID
This is the easiest thing you'll ever have to do within the 29th and can only be a little confusing if you don't know what it is, where it comes from or what it does. This being explained above is just a ride in the park as all you need to do is go to the following link that has the ROID converter and sign in with your steam account. After you've done this you can copy the 'ROID' and there you have it!