[SalesForce] Activity Sharing with a Community User

Each Partner Community user associated with the same account gets a unique Role. When a Partner Community user registers from say, Acme Widgets, that user gets the role Acme Widgets User or something of the sort. Every other user who registers related to the Acme Widgets account also gets that role. Users related to other accounts get their own unique role based on that account name.

Partner Communities have access to the full sharing model, while Customer Communities can only access Sharing Sets and Sharing Groups.

Since each partner user has an account-specific role, it's going to become really tedious to create sharing rules for each partner account. You certain could do what you're asking this way, object by object, one rule for each account. That's a lot of sharing rules. The other option, and it's what I would recommend here, is to create rules using apex manual sharing and a trigger on your object(s) to automatically share it with other users having the same role as the record owner, one trigger for each object. Documentation on apex sharing here:

Sharing a Record Using Apex

In a Private Sharing model with Activities controlled by parent, users can only view activities on the Activity related list if the activity is assigned to that user. Using a sharing rule to open up access on the parent record will not roll down to the activity.

Without any workflow, the the only viable workaround is to change the OWD for the parent object which is not something we would like to do.

Activity field "Public" (On Event and Task objects) is unchecked by default on new activity. This is causing the record not to show up on Activity History section for users accessing shared Accounts.

So one way to achieve this is create 2 workflows one on event and one on task.

Rule criteria: Public equals False
Evaluation criteria: when a record is created, and any time it’s edited to subsequently meet criteria
Immediate workflow actions: Event/Task field update. (Public equals True)

Please note that you might have to update all existing Activity records by setting the "Public" field to TRUE.