Yes, if you goto Keap REST API under REST Hooks you can pull a list with the api of all currently available triggers (in this case contactGroup.applied is what you’re looking for):
The list of available RestHook event types is constantly changing so we don’t provide it statically in the docs; you will want to make a call to Keap REST API to get the current list.
Ok. Another things that seems to be missing from the documentation.
Where can I find the model of the content of every event?
What should I expect from contactGroup.applied and contactGroup.removed ?
The payloads for each type of resthook are different, based on the object type involved. If you’d like to play with hooks you can use tools like http://ptsv2.com/ to set up/receive the hook payload. Below is a sample of the contactGroup.applied, which I ran against one of out test servers: