CS:GO (now available for free) is the full game. New CS:GO players will receive access to all game modes, matchmaking, and a limited set of item drops and weapon cases.
Prime Status is an upgrade available to all CS:GO players. When you have Prime Status you are matched with other players who also have Prime Status and Prime users are eligible for Prime-exclusive souvenir items, item drops, and weapon cases.
There are two ways to upgrade your account to Prime Status; reach Rank 21 by earning XP and add an eligible phone number to your Steam account, or purchase the CS:GO Prime Status Upgrade in-game or through the Steam Store.
As an existing CS:GO user with Prime Status your experience in CS:GO should not be affected. You will continue to play CS:GO with other Prime Status players and receive drops and weapon cases from the same drop lists, some of which are Prime-exclusive.
Additionally, as a Prime Status member you are be eligible for the new Prime-exclusive Danger Zone Souvenir MP5-SD | Lab Rats, available until January 9th, 2019. And if you have played CS:GO in the five years leading up to the Danger Zone update you will receive the CS:GO Loyalty Coin!
All users who already own CS:GO but have not opted into Prime Status have automatically been upgraded to Prime Status. Even if you do not have a phone number on your account, you now have Prime Status.
Yes, but these users will need to find a game from a shared lobby.
When matches are created they are labeled as "non-Prime" or "Prime only". When a non-Prime and Prime user matchmake together their lobby will always be downgraded to a non-Prime lobby; this is done to ensure Prime lobbies and matches are filled with only Prime users. Because of this setting, a non-Prime user will not be able to join a Prime user if they are already in a match that is labeled as "Prime only".
If a non-Prime user tries to join a Prime user's match they will receive the following error message: "Failed to join session because that game server is reserved for CS:GO Prime players."