When making a study plan like joining study groups or just simply setting aside time to study, how often are you able to stick to that plan? Or do you just study whenever you get the chance?

If you don't plan things out and just try to wing it and study whenever you can you may find that you never have enough time to study and you'll risk falling behind in your school work unable to catch-up. Having a set routine worked well for me when I was in school so I would suggest you do the same.

When I was in college I created a spreadsheet which had my entire schedule laid out from when I woke up to when I went to bed. It was very useful because it reminded me of what I should be doing and when I should be doing it. I wrote specific times for classes, studying, meals, clubs, and part-time jobs. I also wrote in when I would take breaks and hang out with friends at parties. 

Writing out a schedule will help a lot because it will keep you focused and things won't fall through the cracks. Having a calendar listing all of the due dates for projects, reports and tests is also effective.  If you do these things I'm certain that you'll be able to stick to your study plan.

It is all about making your study time your priority #1. When you commit to a study group, others in the group are depending on you to be there each and every time and to be on time. It's a commitment to your future. I studied in 2 different groups and carried flash cards with me wherever I went. I would use my flash cards at red lights or while waiting for the bus. It gave me a point of reference when a question was raised in the classroom or on an exam.