My solution is to use shared variables and strings. You stated that there is a grouping by the month. I would put something like:
shared stringvar thisMonth := "";
""
in the group header. this initializes which entries were added this month.

In the detail, my formula would look something like this:
shared stringvar thisMonth;
shared stringvar overall;
if instr(overall, {table.field})=0 then(
thisMonth := thisMonth + {table.field};
overall := overall + "," + {table.field};
);
""

This finds the new/added entries, and updates what the list of entries is.

Finally, in the group footer, we need to find the drops:
shared stringvar overall;
shared stringvar thisMonth;
shared stringvar dropped := "";
local stringvar array allEntry := split(overall, ",");
local numbervar i;
for i=1 to ubound(allEntry)( //I don't remember the syntax off the top of my head
if instr(thisMonth, allEntry) = 0 then(
    dropped := dropped + "," + allEntry;
    allEntry := "";
)

);

overall := join(allEntry, ",");
""

this will find the dropped and reset the list to start the next month.

Finally in the displays it would be simply:
shared stringvar thisMonth //this shows the adds

This should give a roadmap of how I would approach the issue. It should work. It might need some tweaking.

HTH