On most planets (including Basin, where you start) the guild is a small room with a terminal and a Guild Master behind a counter. You use the terminal to browse and accept business opportunities, but this is only possible after you've joined the guild for an entrance fee by talking to the Guild Master.
Guild Master's propositionEdit
"Hi, how are you? I'd like to invite you to join our guild. We broker for the most reputable clients. That means we guarantee your work and your compensation. The Merchant's Guild primarily schedules cargo runs. It will only cost you 4000 credits to join our ranks. How about it?"