No, this is only possible for your own via the event EVENT_MOUNTED_STATE_CHANGED
If anyone in group dismountes/mounts there won't be fired any event or callback.
You could check that with an EVENT_MANAGER:RegisterForUpdate every 1 second e.g., but this may become performance hungry
https://wiki.esoui.com/Running_LUA-C...isterForUpdate
But: I think there is no API to check for IsMounted(unitTag), there only exists IsMounted() for the own player, isn't it? So this would not be possible at all as it seems.
If you are grouped there always exists the way to send small data between players "who use the same addon" ia map pings. But this is a workaround that quite a lot addons use already and if you add to that emthod the server might kick you for server message spamming! So I would only do this if really needed.
At best read about it here at the lib LibGroupSocket:
https://www.esoui.com/downloads/info...oupSocket.html
This way each addon where an EVENT_MOUNTED_STATE_CHANGED fires could broadcast that info together with the unittag's index (2 -> which makes "group2" as unitTag in the end) and teh mounted state (1 for true, 0 for false e.g. -> use 1 here) to the other grup members and the ones using the same addon could use the index 2 -> create "group2" and then know that GetUnitName("group2") has mouted (state 1) currently.
But as I said: If 1 group members all mount at once and all is send to each other because all use that addon: You might get kicked which is not that nice within a raid/trial e.g. So this is nothing one should use for every aspect and nice "feature".