If you have a group of regular customers that historically have attended one type of class, and you're looking to message just that group, you can target everyone thats ever taken this type of class by doing the following:
Create the campaign sequence
Create a campaign sequence
Set the trigger as Class Signup > choose that class
Add an action of Tag > choose which tag to associate
Enable the sequence
Create the segmented list
Then, create a Segmented List that filters by that tag
Click Customers > Customer List > Customer Segmentation
Filter by Customer Tags > Choose the same tag > Apply Filter
Save Segment > choose Dynamic or Static > Save
Select List > Contact customers