For this a few steps are needed to get to this:
- Go to https://make.powerapps.com.
- Open the "Model Driven App".
- In the Upper Right Corner, click the "Settings" Button.
- Select Advanced Settings.
- Then click in the top bar on Settings->Customizations->Customize the System.
- In the menu on the left select Process.
- Browse below and click on "Send Invite"