There are two options to add a job to an existing group: From the list of jobs or using the Duplicate button in a job.
From the list of jobs
In your list of jobs, you can drag and drop jobs using the icon (the one with three lines) at the beginning of each line.
To add a job to an existing group, you simply drag and drop a job into another job (which is already in a group). The system will ask you if you would like to add the job to the group (meaning that it will become a parent job in the group) or create a dependency (meaning that it will depend on the parent job).
Using the Duplicate button
In your jobs, you can use the Duplicate button to add the duplicated job to a new group, to add it to an existing group or to create a dependency.
If you choose to duplicate the job and add it to the group, the job will be added to the group of the job you are duplicating.
If you choose to duplicate the job and create a dependency, the duplicated job will become a dependency of the job you're duplicating.