When you share your screen with someone, they can click to highlight areas on your screen. You can also allow them to control your screen, which gives them complete access to your computer.
WARNING: Only allow people you trust to control your screen. They can do things you can do on your computer—open apps, create or delete documents, copy files from your computer to theirs, and so on.
Go to the Messages app menu bar, then click Allow Control so that it turns blue.
To take back control of your screen—but still let the other person view it—click , then click Block User. You can also click conversation for the person whose screen you want to see.
Do one of the following:
In a conversation with just one other person, click , then choose Ask to Share Screen.
If , and make sure your contact card for the person includes both Apple Accounts—the one they use for iMessage and the one they use for iCloud. Then try sharing again. (See in the menu bar, then click Turn Mac screen sharing on or off.