Waiting lists are not only available for single classes but also for the whole course.
In order to add one or more people to the waiting list of a course, you will need to make sure that the waiting list is enabled for that activity (Settings -> Activities -> Edit -> More -> Waiting List)
Click on Calendar - Course Availability 
Once you have selected the filters to find the course, the system will provide the results in a table on the right. Click on the availability of the desired course.
A new table will appear. Click on the button 
to add people to the waiting list for that course:
Type in the name/s of the people you intend to add to the waiting list and click on Add.
There are two options available:
- waiting list for a specific course (e.g. swimming course on Thursdays at 6PM)
- waiting list for the whole activity (e.g. the customer will be on the waiting list of all the courses related to that activity. So it doesn't have to be a specific course at a specific time on a specific day)
Once the customer has been added to the waiting list, it will also show in the user's profile: