allow someone join in between.

say you have a course for 10 days (1 email each day) and some one joins on day 5.
easier option
send him not just 5th days email but all 1 to 5 days emails so far.

harder option (it can be in addition to easier option)
Instead of he/she getting day 5 email, allow to get starting from day 0. So when everybody else will get 6 th email, he can get 2nd email
this can be done attaching a count to track how many days since user joined. and send him the email matching tracking count. so whenever he joins he gets day 0 email. (when someone else may be getting day n email)