The announcement was made via their official Youtube channel.
Honestly I was surprised at the announcement as Chu-Z has never come off as the kind of idol group that would make additions to it. It makes me wonder if they want to stay as a six member group or if there may be another graduation in the near future. I hope it is not the later though with Maia turning 28 this year I would not be completely surprised if she leaves. But I do truly hope I am wrong and they just want to have six members.
As things are now while the group as a whole is still strong after Asuka's recent graduation the sub-units do feel off. Which is not surprising as going from three members to two does make more of difference than from six to five as there are less members to cover for the missing member. So wanting to add a member to bring Sun Chu up to three again makes sense.
Overall my feelings are mixed about the announcement. While I am willing to welcome the new member it just feels a bit off still to being one in. And then there is the dread that it is more because someone else in the group has graduation in mind and the agency does not want the group to get any smaller than it currently is.