The key sharing only works to the address that is entered.
For my example assume you have a Hotmail account which is registered with AxCrypt and a Gmail account which is not registered with AxCrypt.
AxCrypt needs to know which recipient is authorised to open the file therefore if your friend is emailing your Hotmail account then you’ll be able to open the file assuming you’re signed into AxCrypt.
If your friend emails the Gmail account then you’ll not be able to open the file unless you accept his invitation to ‘join’ AxCrypt for the Gmail address. You could set the same password if you want (to simplify things) because only you need to know the password.
But if complicates things for him and you having two email addresses sharing things to and fro because you’ll have to Sign Out of AxCrypt and then Sign In each time you want to view the file using the different email!