When you're in a workout, you'll notice a double-person icon in the upper right header bar with a blue square next to it with a number in the middle. The number represents the number of people in the class with their cameras on (you need to have your camera on for people to find you).
When you click on the person icon, you'll get a pop-up menu from the bottom-center of the screen. This is where you can select the friends you want to see in the right-side bar or in "Party View".
To select someone, you'll click on the camera icon to turn it green. To "Friend" someone, click on the person icon to the right of their name and they will automatically populate your side bar friend section when then join future workouts.
Pro tip #1: The only athletes that you can hear in class are the ones you've selected.
Pro tip #2: In order for your friends to hear you, you need to click the mic icon in the header to green (remember to click it off again unless you want your friends to hear you heavy breathing and your dogs barking).
Here are your viewing options:
"Side": When you select the side option in the friending pop-up, you'll be able to chose up to three friends that you can see on the right side of your screen.
"Grid": This option allows you to select up to eight friends who will fill the center of your screen, and the coach moves to the upper right.
"None": Clicking this option will restore your screen to you and the coach.