The personal portfolio doesn't allow adding collaborators.
If you're a representative of an investment firm or investor pool, you can easily create an investment and move the investments from your personal portfolio there.
This unlocks professional investor features like collaborators.
Part 1: Setting up the investment firm
Follow the steps on How to create an investment firm
On the left sidebar, click on Home
Then, again on the left sidebar, click Performance
Here you will see the companies you have your stake. Hover over the right side of the row with the company and click on the 3 dots.
Click on "Move investment" and select the previously created firm.
Repeat for all companies you would like to move to the investment firm
Part 2: Inviting collaborators
Follow the steps in How to invite collaborators to Investment Firms.