Hello
I saw only 1 topic discussing this with a recommended plug-in, but I’m wondering if I can do this myself.
Idea is to modify the opportunity module so the relationship with users is one-to-many (opportunities being the “left hand side” module and users being the “right hand side” module). Is it as simple as that or I’m missing something?
Thank you
Standard logic is “one assigned user” - you are right Security Suite Pro allows this included
You have the option to add a custom relationship between Users <> Opportunities
Go to Studio
Users > Relationships > Add Relationships > One to Many with Opportunites
After you have done that go to Repair - Repair Relationships & Quick repair and rebuild
In the Opps record you will still have the “Assigned to” - You could potentially change this to “main user” or something
You will also now see a “User” subpanel in the opportunities record
One downside of this would be the Opportunity list view will only show the “main assigned user” you would not be able to see the “many users” assigned to the record from list view
You could create a Report in AOR to display this 
Hope this helps