I know about the Charites, the goddesses Aglaea, Euphrosyne, and Thalia. But there is apparently a younger Charites group?
Like much of greek mythology, it all seems to be a bit of a mess and it depends on which long dead spirit of an ancient poet you ask. There is disagreement on both the number of Charites/Graces and their names.
The poet Nonnus, on the other hand, names the three Charites as: Pasithea, Peitho and Aglaia. So we lost Euphrosyne and Thalia, and gained Peitho. The Spartans were more spartan, and made do with two: Cleta and Phaenna.
And then there are the daughters of Aglaea and Hephaestus: Philophrosyne, Euthenia, Eupheme, and Eucleia. Who are also considered "members of the younger Charites" (according to wikipedia, but lacking clear citation)
So depending on where you encountered the term "younger Charites", it might refer to the daughters of Aglaea, or any of the younger members of the Charites (however many there were - and which may or may not include the daughters of Aglaea).