Create Parameters for Member Filters...where to start?

I am sure there are many ways to tackle this.  One approach would be to use a text property on the accounts to tag the accounts you want grouped together, with the same text field.

for example, tag your IC A/R accounts (including related plug) with Text1 = ICAR, tag your IC A/P accounts (including its related plug) with Text1 = ICAP, and so on.

Then in your parameter set up a delimited list such as this, again just an example:

Then in your cube view use that parameter in a Where clause like this:

When the user runs the cube view they will be prompted to select which group of intercompany accounts they want to see. Once they select, that is used in the WHERE clause on the cube view to pull back only those accounts.

Again, this is one approach.  I am sure there are many other approaches.