This is interesting.
When a Grouply user goes to their Invite Groups “feature” (spam gun), they are shown a list of all the groups where they hold membership. Next to each one it says whether the group has been spammed yet. For those already hit, it says:
“Last told about Grouply by [subscriber personal name] at [time] on [date]”
Too bad it doesn’t provide the sender’s email address. But that usually can be found on their Grouply profile.