If you're having trouble inviting users, make sure you're the account's owner or an admin user. Furthermore, Contacto does not support personal email addresses (such as those with the gmail.com or outlook.com domain), so you cannot invite someone with a personal email address to your account.
If a user does not receive an invitation, request that they check their spam folder. You could try sending the invitation again. If you continue to have problems, contact our support team.