All professors and GSIs are required to have office hours during the week, usually 1-2 hours at minimum. Office hours times can be found out on the first day of class, on class syllabi, the internet, and at some point, this wiki.
Depending on the popularity and difficulty of the class, there might be a lot of people showing up at office hours. If you are looking for serious study partners, this is an excellent place to find them. People who attend office hours tend to be willing to go the extra mile to do well in their classes.
Office hours are a chance to ask questions about lecture, homework, upcoming tests and even to just get to know the professor/GSI. It is not even necessary to have questions of your own (though it is advisable), you can simply sit in at office hours and listen to answers and explanations to other people's questions. Office hours provide the opportunity to talk to and get to know the professor/GSI on a personal level, and for them to get to know you. Developing these relationships can pay off down the line in the forms of advice, letters of recommendation, and even research positions or jobs.
Even though it can be tempting to skip this extra "class time", the things that can be learned and the networking that can be achieved during office hours are invaluable for those who want to get ahead.