Lists and smart lists can be shared and referenced across workspaces for easy reuse from the Database. 

Person partition rules apply (smart lists and static lists in a workspace only show people who are both members of the list and members of the current workspace).

Share a List or Smart List 

Go to the Database.

Right-click a campaign folder. Select New Folder.

Assets can only be shared across workspaces if they are nested in a folder.

Drag and drop a list or smart list you want to share into the new folder.

Right-click New Folder and select Share Folder.

Choose a Workspace to share with. Select Save.

Awesome! This list will now be available in both workspaces.

In Marketing Activities, you can only share top-level folders. In Database, you can share top-level folders as well as one level down.