Sharing an excel sheet with external users

Hello Clements365 

you can break the permissions on a site, library or file. You can give write oder read permissions for a defined group.

Here are more information about permissions:

https://docs.microsoft.com/en-us/sharepoint/customize-sharepoint-site-permissions

Or here is a good explanation:

https://sharepointmaven.com/sharepoint-permissions-simplified/

Please attention: I recommend you, that you break on higher level like site or library, only in exceptions on file level.

Regards, Dave