SOLVED 😀

I solved this by adding an additional column to my report called, for example, "relationship set".

The value of relationship set is determined by an expression:

[font="Courier New"]=JOIN(LOOKUPSET(Fields!Accnt.Value,Fields!Accnt.Value,Fields!matrixCol.Value,"Main"),", ")[/font]

Where "Main" is the dataset that you are using (I am only using one dataset so looking up itself).

The performance seems to be great too even with large datasets.

The results for this example would be:

Accnt Relationship Set

1 B

2 B

3 A, A, B

4 C

The relationship set can then be used to group common values within the matrix.