Group classes are one instructor teaching 4 to 20 students, the same material, at the same time.
Usually men stand in one row, the ladies stand in another row facing them. (think red rover without the running, but all the fun)
The instructor will demonstrate and teach the men their part of a pattern and repeat the step a few times in front of them to practice it with the men. Then he/she will show and teach the ladies their part to compliment the man’s step, and practice it in front of them. Finally the dancers pair up and practice the step together with music.
At this point the teacher will usually add in tips and hints to make it look and feel better to you and your current partner.
Then on to the next pattern, step, or technique for that class.
Often the instructor will give you the option to “rotate” partners. I know this seems frightening at first, but the experience is invaluable. Even if you plan to dance the rest of your life with only your spouse, feeling the different types of lead/follow, tone etc. gives you a better understanding to help you improve your own dancing. It also allows you to meet other students at your level and create new friends at the studio.