Wouldn’t be best to use GPG to sign all the notes and so if we want to share them we could do it without un-encrypting them but instead giving our key to the recipient so they can decrypt the note?
@darkID welcome to the forum.
The way your question is phrased I assume by "sign" you mean encrypt?
Apart from the practicalities of implementing this (mainly, Joplin itself would not be able to read the encrypted notes), that's not how GPG works. You have to encrypt the note with using the public key of the recipient. Therefore if Joplin was somehow able to manage this you would have to encrypt all your notes with the all the public keys of everyone you may wish to share a note with and that could be a different list for every shared note. Of course if you were to encrypt the notes just for yourself and circulate your own secret key there would be little point in encrypting it in the first place.
It would likely be easier just to copy the note to the clipboard or PDF export it, GPG encrypt it using the public key of the person you want to send it to and email it to them!