If you are an Admin for this partner ID, you can navigate to Org settings from the left panel. Click “People” to add a new user—
Click Invite new member.
Enter e-mail and role type. Select the product configurations to give access to on Sandbox and/or Production. To understand what a “role” is, read this.
Once done, hit Send Invite—this invites the user to log in to your partner ID with the above rules.
Here’s a video walkthrough to give a “Reporter“ access to a product config on Sandbox.