engagement program question

This is the first time I'm doing a continuous type of engagement program where a specific segment will be sent an email once a week until they do a certain action.  But the following month, I'll be adding a new set of emails, and a new group of people will also be added to this program.  

The new group of people I don't want them to receive the very first emails which takes them to the March content on our website.  I'd want them to start receiving the April emails only.  How would I set this up or isolate the new content to be received only by the new group of people.