If the user you want to invite already has an account on Waydev, he can't be invited. He needs to delete the account, and after you can invite him.
1. Go to Project -> 'Users'.
2. Send an invite to the user you would like to add.
3. The invited user will receive an invite from Waydev. He needs to accept the invite in order to collaborate on your account.
4. Select first name, last name and add a password.