In a workgroup, there is no centralized authentication, so each user that needs access to a shared resource will need a local account on the machine that is hosting the share.
In your case, if your friend logs on to their laptop with a local account, for example Bob, and then tries to access a share on your laptop, your laptop doesn't know about the account 'Bob' and will deny access.
You will have to create a local account that your friends can use to access your shares. You can create local accounts with the same user name and password, just understand its not the same account that is on their laptop. If, for example, they change their password on their laptop, this will not change the password for local account you created for them.
Don't forget to assign NTFS permissions to the new local accounts you create over the shared folder. They will need both share (which you've already assigned everyone full control) and NTFS permissions to access the shared resource.
Hope this helps,
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