A quick guide to the difference between Kit and group.
KIT is also called as a bundle or combo of different products. While buying the KIT customer needs to buy entire KIT.
For eg, if any seller selling electronic products then he can create a KIT of Mobile Phone with earphone and Back-cover. In this case customer has to buy entire KIT includes Mobile Phone, Earphone and Back-cover.
Group is a variation of single product based on different option type such as Color, Size, Type. While buying customer can buy single product from Group.
For eg. if any seller selling apparels then he can create a group of color or size variation of T-shirt such as small, medium and Large size T-shirt. In this case customer can but only small size T-shirt from group.
Hope this answers your question.