First you will need two things: a new Unity project and the email address of the person you're adding to the project.
When you have both, open your project and click on the Services tab in the column on the right side of the screen.
This will allow you to create a Unity Project ID for your new project. Below SERVICES, click where it says Select organization. This will show you a list of organizations to choose from. You should only have one, an organization with the same name as your Unity account.
Click on your organization name. This will cause the Create button to light up. Click on that button.
What you see in the window will change to look like what you see below.
Next it's time to upload your project so your collaborators can pull it. Click on the Collab button in the upper right of the Unity Editor. It's right above the window displaying the Unity Services.
You'll see a button that reads Start now! Press this button.
Unity will do some things, and then the Collab button will change to have a blue arrow on it.
This means you can push your changes! Start by pushing the blank project. Click on the Collab button to open the window where you can push updates. Type a note in the window and then click on Publish now!
Now your Collab button should display a green check mark.
Your project is checked in and ready to go! Now you can add a collaborator to it. Open a browser window and head to http://unity.com.
You'll need to log in by clicking on the circle in the upper right hand corner of the screen. Once you're logged in, click on the circle with your initials in it in the upper right hand corner of the screen.
Click on Organizations. This will take you to a page showing your Unity organization. Click on the name of the organization.
This will take you to your organization page. From there, click on Members & Groups.
This will take you to the Members & Groups page, where you can add a new member of your organization. Click on the blue + Add members button.
Next you'll need to type in the email address of your chosen collaborator.
Scroll down from here and click on the green Next button.
You'll be taken to a screen where you need to confirm the addition of a new member of the organization. Click the Invite members button.
Once that's done, you will need to click on Subscriptions & Services in the left hand menu.
This will take you to a Subscriptions section. Here click on the blue Manage seats button.
From the next window, you'll be able to give your new organization member a seat. Look for the name of the person you just added to your organization in the list.
Click in the check box next to their name to highlight the Assign Seat(s) button.
And now only one more step to go. In the left hand menu, click on Projects.
This will take you to a list of your current projects. You should see the new one you created.
Click on the blue name of the project. You'll see some details. From here click on Settings on the left side.
This will display a menu where you can click on Users.
You'll be taken to a new page where you can add users to the project.
From there, click in the field that reads Add a person or group.
The person or people you have added will appear automatically. Click on the name of the person you are trying to add.
That person's name should now appear under Members of this project. They should now be able to see the project in Unity Hub, and download it. You're done!