To play with friends I just use the steam invite, but that doesn't work for 'standalone' servers, which I guess is what you are looking to make. (I didn't know that was even a thing in Valheim)

As for campfires, they so far behave very realistically - they can be killed by smoke, if the ventilation isn't good enough. That sounds like what is happening to you - light the fire, create smoke, after a short while it's killed by the smoke. I recommend not covering it with a 'hooded' roof or a 26' one, but a 45' one.