In "Conditions" section of the User Smart List Editor, click on the "Create New Filter For Smart List" and select "User Engagement Status";
Select a specific action/activity/metric.

A partial list of popular selections for User engagement metrics:
- ActiveContactsStatus - Average interest/engagement level of Contacts belonging to this User
- AssetUsedStatus - Frequency of asset use/activity
- EmailActivityStatus - Frequency of email use/activity
- LoginStatus - Frequency of User logins
- ...and so on...
A user may filter based on the frequency of these and other activity types.
When you have made your selections, click on the blue "Add" button.