Prime Status is a feature of CS:GO for users who have purchased CS:GO or are Profile Level 21 or above. Users who have Prime Status receive benefits only available to Prime users.
Users with Prime Status are matchmade with and against only other Prime Status users in all game modes. In addition, Prime users are eligible for Prime-exclusive souvenir items, item drops, and weapon cases, and have access to all Community-operated servers.
Prime Status can be added to an account two ways:
All users who owned and played CS:GO in the five years prior to December 6th, 2018 have been automatically opted into Prime Status. To verify Prime Status on your account, press the 'Play' icon in the upper left and look for a green coin and 'PRIME ENABLED' on the next screen.
If you have recently reached Profile Level 21 you don't need to do anything - users are automatically opted into Prime Status after reaching Profile Level 21.
No. Currently there is no way to remove Prime Status from your account.
Yes, but only if these users matchmake from a pre-made lobby. When a Prime and non-Prime user matchmake together the lobby is considered non-Prime and the resulting match would be with and against non-Prime users.
It is not possible for a non-Prime user to play in a Prime-only match.