i believe the raid api has hooks for people joining/leaving
It does. I have a mod that uses it. You can then scan for party change and update an array of people when it happens. GRSS would need an additional set of triggers selectable by the user also. A Raid Start and Raid Stop button or slash command. Each of these could do 2 things. Save the server date and time of the event and capture the Raid at that time. On the events that trigger when players join and leave a raid you could then simply compare the current raid list to the raid list at start. Anyone that is new could be added to the Late List. Anyone that is no longer present could be added to the left early list.
Are you wanting to punish these people like deducting dkp for this behavior or just post it like an overall raid summary to show "Who attended the entire Raid", "Who Arrived Late", and "Who Left Early"?
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Six Demon BagJack Burton: Hey, what more can a guy ask for?
Egg Shen: Oh, a six-demon bag!
Jack Burton: Terrific, a six-demon bag. Sensational. What's in it, Egg?
Egg Shen: Wind, fire, all that kind of thing!