Can I join a class that has already started? Can I enroll late?
Usually, although not always - enrollment policies vary from course to course. Some instructors choose to close enrollment early to ensure a complete course experience for students, but most will leave enrollment open for at least a few days - sometimes a few weeks - after the course begins.
You won't receive any deadline extensions if you enroll late, so if the course has been running for a while, you'll want to check the course policies and syllabus to see how much you've missed. If you do find that many of the important deadlines have already passed, you can still "audit" the course - just focus on watching the video lectures and learning more about the course topic, so that you'll be well prepared to complete the coursework in the next session if the course is offered again.