Find out how many people within a stream have opened at least one email.

Which smart list filters did you use?

Member of Engagement, Stream X
Opened Email XYZ at least once
Past X days

should provide you what you want.

Hey Alex, 

You could perhaps use program statuses to help track this depending on how you want to bucket your time frames for reporting. 

If your channel has a status of "opened" and any subsequent status (e.g., clicked) must first pass through open, then you would know that anyone with a status of "opened" or greater had opened at least one email. 

The campaign to change the status would trigger off

Email is opened
Email name contains [engagement program name]

Flow - change status in program to Opened

This one campaign could cover you for any email in the program. 

In your reports you could use "created date" as a filter in a smart list to see only certain cohorts of leads in the program. 

If you are looking to cohort by activity date instead (i.e., the date of the email being opened) I think you are back to smart lists for this. 

Thanks for the help.