Such an RACI Matrix will be much more easy with the upcoming new relationship entity that the fibery team currently is working on (References Reloaded), because now we are still working with read-only lookup fields. This now means that if you create lookup fields for each RACI database item related to a user, you cannot edit the values in the table.
Here is a workaround:
Create an ‘RACI entry’, ‘Sector’, ‘Activity’ databases.
In the RACI entry database, create relation fields to Sector and Activity, plus as many multi-select fields as you have team members. The multi-select fields all have the same options (R, A, C, I)
Name=formula=entity ID, or a custom code you use for the responsibility.
