You may not have a group activity in your workout plan if there is not an activity that corresponds with your "Availability" that you established in the "Goals" section of the member's profile.
However, if you would like to manually add in some activities go the the "Workout" tab of the member's profile and click the settings (gear icon) for the current workout.
In will take you to the following screen. Press the button "Edit"
This will take you to the workout editor. Select the session (week and day) that you would like to add an activity to. Then using the exercise search tool to the right, select "Activity" (it will be green when selected) and type in the name of the activity in the search bar. Then drag and drop the activity on the day you wish to schedule it. (Please note: depending on the validation of sessions by the member it could be that the activity is not taught on the same day that the session you assign it to occurs)