Hi Mark,
As I understand it, it's pretty straightforward. If a lead enters an engagement program -- let's say Stream 1, which has 5 emails in it -- they start receiving those emails according to the stream and the cast schedule. Marketo checks to see if that lead has been sent that email before (via the email id). Delivered is not relevant, since Marketo can't control that (it's dependent on the message sent back to Marketo from the recipient's email server)....The only logic that is different in an engagement program is this "check" for email being sent to this lead. If the lead HAS been sent this email, then they are sent the NEXT email in the sequence that they haven't been sent....
For program membership in an engagement program, all you need to do is create a campaign in the program that includes the "add to engagement program" flow step and then specify the streeam.
Hope that helps,
Jono