Have you tried setting the element only d-List as a time dimension, the Groups as an additional d-List, then setting up an assumption d-cube with a second dimension of three elements:
Subtotal (=1)
Cumulative count (Cumulate Subtotal)
Group formatted element: Conditional Group (if Cumulative count <50, Group1, if {Cumulative count} <100, Group2, etc.
From this assumptions cube you can generate a d-List with the subtotals included, and generate the e-list from that.