Users dont recieve an email when a user is added with wp_insert_user

I am creating a website for an client when people can come on buy courses and the user get access to the courses i am using s2member to control the users.

The issue I am having, When an person buy courses they need to add the users to that course i am doing this by them inputting their name and email and mass adding them, the issue is when they are added they get a random password. Te big problem word press does not send them that email.

Now when you make an new user account in the admin area, there is an option to email the login details to the user. What i want to know is how do i force that on so when i user wp_insert_user it works or what function i have to call to make that work. Or should i just use the php mail like word press does and send it myself?