Got it !!!
Here is the code... you also need a CharacterMakeNPC to set them back to a non-player on leave. (keeps it all clean)
IF
ObjectFlagSet("Galwin_InParty", _speaker, _)
THEN
CharacterMakePlayer(CHARACTERGUID_HK_Companion_Galwin_42c0121f-c906-416d-abaf-9b9e91f17b65);
ShowNotification("Galwin Joined Your Party");
IF
ObjectFlagSet("Galwin_LeaveParty", _speaker, _)
THEN
CharacterMakeNPC(CHARACTERGUID_HK_Companion_Galwin_42c0121f-c906-416d-abaf-9b9e91f17b65);
CharacterRemoveFromParty(CHARACTERGUID_HK_Companion_Galwin_42c0121f-c906-416d-abaf-9b9e91f17b65);
ShowNotification("Galwin Left Your Party");
I shall make a tutorial on this once I have 5 mins.