In our sales, we often have several stakeholders involved in the closing of an opportunity. Even though there is technically just one customer. We often need to engage with several different accounts to progress a deal forward. As such, it is important to track what accounts are stakeholders of each opportunity. A given account can be a stakeholder to many opportunities and a given opportunity can have many stakeholders. Thus we set up a junction account to link Accounts and Opportunities to represent the stakeholder relationship. So we will use the normal Account - Opportunity relationship to represent the customer and the junciton relationship for stakeholders. We also track tasks at the Opportunity level. For any given "Stakeholder" Account, we want to see all the activities associated with Opportunities for which it is a stakeholder. For example, lets say Account A is a stakeholder for Opportunity X, Opportuity Y, and Opportunity Z and each of those opportunities have 5 tasks Related To them. We want to see all 15 tasks shown in one place. We have tried dozens of iterations of reports and we can show the Account - Opportunitiy relationship and the Opportunity - Activity relationships, but can't connect them together. We don't even need it to be a report, any way we can show them all together would be useful. Any help is greatly appreciated.
#Reports & Dashboards #Junction Object #Activities #Opportunities #Accounts
Vuk Stajic (MVRK Inc.) 
Forum Ambassador
A more simple approach - and perhaps you tried this and it did not work for you - would be to use the standard Opportunity Contact Roles feature for all involved Contacts regardless of Account.
And to have a Contact Role or Roles that define those individual's responsibility/task/relation to the opportunity.
Then you have a single list of who is involved in what opportunity, even though they may not be contacts of the Account related to the Opportunity.
And, you can set all tasks in the single Opportunity record even if they are call notes or meetings etc with contacts not a part of the Opportunity Account.
More reading on Contact Roles: https://help.salesforce.com/articleView?id=sf.contactroles.htm&type=5
At a company I worked at I would sell services to an account, but would have 1 or 2 or more partner organizations selling products to the same account at the same time.
In my opportunity I would track all of the Contacts from my client involved, but also track the other Account Execs and Solution Engineers and VPs etc of my partners involved in that Opp.
Happy to chat more about it if needed :)
