Hello,
This will not be an issue. It would be the same as if you had added multiple normal keys to the fileshare. When you add the group key, There would be a change in the metadata for the fileshare, but it would not cause the entire fileshare to re-encrypt. As long as the fileshare is set up properly, and has a normal key as the admin. From the information you gave, this sounds like a supported configuration.
As with any encrypted data (and data in general), backups are always important.
Cheers,
Phil