If, when you added users, you chose to withhold the invites, or you have a user who didn't receive theirs, then you can resend.
(Note: this function is not available on mobile)
You can identify the users who've not yet logged into Timetastic and created themselves a password by the sad face against their name in the USERS page.
It's these users you'll be able to resend the invite/ welcome email.
To the far right of the users screen you'll see the 3 dot for additional options.
Click and in here you'll be able to resend the invite.
The invite is sent immediately.