Hi there,
It looks like GroupInvite(unitTag) is delayed after combat or is it the recipient prompt that is delayed after combat?
The real question behind that is :
Can a group invite being sent during combat and accepted by the recipient (with a filtered AcceptGroupInvite() in an addon) before the end of the combat?
As one is not able to use the interaction wheel menu to invite players during combat I guess it is not possible to do so via the UI, so also not via the API during combat.
At least I never got any group invite during combat.