Here is an easy way that might just work:
1. Create a self relationship using the grouping ID field
2. use "Replace Field contents" on the related table.
of course that would include all records not in your found set with the same grouping id.
Well you have to start somewhere.
I would do it like this:
New Window (beyond screen bounds)
Find Matching Records by current groupID [ Constrain ]
Replace Field Contents
I'm not sure but if the value is always same on records having same grouping ID, the value could be saved in a record of another table.