The book certainly doesn't answer this question; we don't have a record of him getting on the lifeboat.  Pi, when he gets on, notices the other animals-the hyena and the zebra.  But, he doesn't notice Richard Parker until later on.  So, he was on the lifeboat already when Pi got on.  It's a good question that you ask though.  What I figure is that the other animals must have been on already, to counter-balance the weight.  The weight of the other animals served to counteract the heavy tiger climbing in.  It was probably a very tumultuous affair; lots of upheaval and water in the boat, but he managed it.  Throughout the book, you will find yourself asking similar questions quite often, questions like "How in the world was THAT possible....?"  But, Pi ends up explaining most of them.  Martel, in interviews has admitted that he put pretty unbelievable stuff in there on purpose, so that people would have to take a leap of faith to believe it; he equated this to religion, which requires faith to believe in it also (which is why he spent so much time discussing religion at the beginning).  So, believe it, try to imagine it happening, and just roll with it and enjoy the book.  :)  That was my attitude, and I liked it a lot as a result.  I hope that helps!

Very carefully. Actually, you have to remember this story is a fantasy and things which seem impossible and irrational can happen in a fantasy. One of the things that an author must do is to carry the reader into a world where he or she believes a wet tiger could get into a lifeboat without capsizing it. Perhaps the author was not successful in getting you suspend you belief system long enough to believe in this story. However, if you try to use your imagination, I think you will be pleasantly surprised by this story.