Building Jenkins pipeline with GitLab info
Gitlab Subgroups are essentially only additional path identifiers on the Gitlab server.

For example if you have:  


Gitlab server on localhost   
Group: test
Subgroup subgroup1  
Repository myawesomerepo


Then the path to your repository will be:

http://localhost/test/subgroup1/myawesomerepo


And cloning the repo will be

git clone git@localhost/test/subgroup1/myawesomerepo.git


So when you connect any Jenkins pipelines to the repo you will need to connect them to that full path.  

Any changes being done or API's added to the Git repository itself can easily be monitored automatically to do some action via Jenkins.  

However, since a Gitlab Subgroup is not a Git feature rather a Gitlab feature, you need to try and write you own logic to query on the full URL and to see if there were any new Subgroups/ API endpoints added.