Let’s say you are working on Dynamics 365 portal dev environment and you want to quickly add your demo users. You can following below instruction to add users without invitation.
1. Open to contact record whom you want to add as portal user
2. Make sure you are on the Portal Contact form
3. Navigate to Web Authentication section

4. Add username and set Login Enabled to Yes, leave other setting as default

5. Click on CHANGE PASSWORD and set password for the portal contact

Done !! now this user will be able to access portal.

**Make sure to assign required web roles to contact.