Q: 1. What is the source of the Urantia book? Who wrote it and where did the extra-biblical information come from? 2. What is the means by which a person enters into relationship with God? Is it automatic for everyone, or is there something that a person must believe or do? 3. Who is responsible for authorititively interpreting the teachings in the Urantia book? Is there an individual or group who is responsible?
Thank you for your post to Truthbook.com. Yes, this can be confusing on first contact -- I'll see if I can help.
For background, The Urantia Book speaks for itself; it claims to be the next (the fifth) revelation of truth to the world following on the heels of Jesus and as such it proclaims that nothing that has transpired in the intervening 2000 years is of the epochal significance it presents to you personally. There are organizations that publish the book and handle administrative tasks related to the shepherding of the revelation but there are no official pronouncements about the book's contents other than those the book makes regarding itself. As a new revelation of divine truth it is written specifically to you, not to any one culture, group, religion, race, but to each of its individual readers. It is a revelation of truth in book form with no authorized human spokesperson.
With that background the answer to your first question is -- the authors of the 196 Papers (or chapters) of the book plus its foreword are listed and described within the text of the book -- we'd call them celestial beings because they do not live here (on Urantia) -- they were tasked with a divine mandate to present the new knowledge and spiritual insight that are contained in the book. This occurred in the 1930s and was completed by the early 1940s; the book was first published in 1955. Some of the celestial beings either are, or are in close contact with, the angels administering life on this world -- the extra-biblical information you mention comes either from previously published or written records, or, when those were inadequate, it and other revelatory material comes from the records the angels keep.
Your second question: an excellent and insightful question. There is no teaching, no philosophy, no religion that is required in order to enter into a relationship with God. Teachings, religions, philosophies can help, can provide stepping stones and bridges to know and understand our heavenly Father, but any human being, even one born and raised in the jungle, can have a personal relationship with God -- God resides within every human being (in the Urantia revelation this divine fragment of the heavenly Father is named the adjuster, thought adjuster, mystery monitor). God is right here within us, we each have an inborn urge to seek God in whatever manner we may and if we do so a connection is made. Meditation may help, prayer and worship may help, a tragic or life-altering event may help or the connection may just come naturally. However natural this connection may be, it isn't automatic or automatically experienced because many people don't recognize its existence. God reaches down to us but we are required also to reach up toward God in order for that connection to occur.
Your third question -- who is responsible for authoritatively interpreting its teachings? You are. No one else is. The book is written to you -- your challenge is to read it. If it rings true your life will be transformed. If it doesn't then it would be your duty to refute its teachings -- that's something many attempt to do without reading the book or understanding its message so such criticisms are irrelevant. There are thousands of students of The Urantia Book who would be willing to discuss their understanding of its teachings with you but the only way to know the truth is to taste and experience if first hand.