Resending a welcome email to new users
You can resend a welcome email to new users within Settings > Users, by clicking the 'email' button to the right of the user.

In the final column you can see whether the user has been sent a welcome email previsously (it will tell you the date it was sent).
If it says n/a this means that no welcome email was sent when the user was created.
Users who don't have an 'email' button next to their name have already created a password and logged in.
Therefore you can't resend them the welcome email!
If they have forgotten their password, they need to reset their password themselves here.