To my knowledge connecting from one machine to another while in PIE is not supported. The reason is that the “map package” that you’d have is a temporarily saved version of your level at the time you started PIE. Even if the other user has the same map, they don’t have the exact same map path/GUID wise. If you got further it will tell you that you don’t have the same content and fail.
If you don’t chose an online platform (Steam, etc), you should be using the OnlineSubsystemNULL interface and you should still always get a valid, albeit fake, UserId. It would look like a GUID with the machine name in it (see GenerateRandomUserId())
That being said, the user id Invalid message you see shouldn’t be a big deal. It is simply logging the NMT_Login phase of handshaking. I’d expect more errors/info above and below that indicating the actual error (which may be what I described above). The client logs from your friend would probably show the right thing, but including both logs would be best. Can you unsuppress DevNet and DevOnline logs?
DefaultEngine.ini
[Core.Log]
LogNet=verbose
LogOnline=verbose