The park allows thru hikers to show up and stay at the Birches without reservations because it's not practical for thru hikers to do so. Baxter is under no obligation to make arrangements for your friends and/or family and why should they?
There are a number of places your friends/family can stay just outside the park, like Abol Bridge campground. Getting a ride out is no problem, you use the phone at Abol Bridge campground, but no doubt shuttle drivers are there every morning during the busy season anyway.