1. Install the app from the AppExchange. We recommend using the Grant Access to All Users option when installing, you can customize certain profiles later.

 2. Navigate to Setup > Object Manager > Opportunities > Page Layouts
(If you're in classic, Navigate to Setup > Build > Customize > Opportunities > Page Layouts)

 3. Edit the Opportunity Page Layout you want to change.

 4. Add the Opportunity Team Edit button to the page layout. If you've overridden the predefined Lightning Actions, also add the Opportunity Team Edit action to the page.

 5. Save.

 6. Finally, make sure the Visualforce page is shared to all profiles that need access. If you chose to Grant Access to All Users when installing, then you can skip this step. Otherwise, you can add or remove access by editing a Profile and removing access to the OpportunityTeamEdit Visualforce page.

That's it, you're done. Now go open any Opportunity that you can edit and click the Opportunity Team Edit button to edit the team members. 

Check out the Overview for more information