Get in on the ground floor - this means joining in to something at the beginning of it - be it a company with growth potential, or stocks, et. Get my foot in the door - is what you do when you want to be part of something but don't know how to get in or are not ready. For example - if you wanted to work for a great company but they were not hiring you might offer to work for free for a couple of weeks or a month to get your foot in the door. You are convinced that when they know how good you are they will want to offer you a full-time job. They are definitely not inter-changeable.