Pin a conversation in Messages on Mac
With macOS 11 or later, you can pin specific conversations to the top of the Messages list, such as the people you contact most.
In the Messages app on your Mac, select a conversation.
Do any of the following:
Drag the conversation to the top of the sidebar.
Control-click the conversation, then choose Pin.
To unpin a conversation, drag it out of the section or Control-click it, then choose Unpin.
If you receive content that’s especially interesting and Shared with You is on, you can quickly pin it in Messages, and it will be elevated in Shared with You, Messages search, and the Details view of the conversation. See Pin shared content.
With macOS Ventura, you can also pin a specific project you collaborate on. See Manage a project in Messages. Pinned items appear in the conversation Details view.