You can customize the welcome e-mail that is send to trainees.
There are two e-mails. One when a new account is made and one with the invitation for the program.
Go to account => Emails
Here you can customize the email to new users.
When you enroll a trainee in a program, an invitation email is send automatically. You can customize this email in the following way:
In the program window click on "Edit"
Scroll down to the bottom. Here you can customize the invitation email.