Originally Posted by Masteroshi430
Firstly be aware that function doesn't return the correct data when you are too far from the group member.
Secondly it still works properly in minimap by Fyrakin.
Try to verify what GetGroupUnitTagByIndex outputs because it does weird things sometimes. 
EDIT:
Sorry I read too fast, I'm only using the 1st value returned and it works correctly, I don't know for value 2 and 3
GetTargetMountedStateInfo(*string* _characterOrDisplayName_)
** _Returns:_ *[MountedState|#MountedState]* _mountedState_, *bool* _isRidingGroupMount_, *bool* _hasFreePassengerSlot_ |
It used to work perfectly. Ive had my addon setup to handle "saved" player" not being mounted on dual mount and with a passenger already using this function even when out of range but now its returning true for every single group member in testing when noone is mounted at all, inrange and not inrange.