There are two options to create a 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 create a new group, you simply drag and drop a job into another job. When doing so, the system will show you (with green arrows) in which jobs you can drag and drop a job. It will then ask you what you would like to do:
If you choose to create a new group, both jobs will be "parent jobs" in the group (you see that they are "at the same level").
If you choose to create a dependency, you see that one job is the parent job, and the other one (the Revision in the screenshot below) is a "sub-job" depending on the main job (what we call a dependency).
Using the Duplicate button
In your jobs, you can use the Duplicate button to add the duplicated job to a new group or to create a dependency. (If the job you want to duplicate already belongs to a group, you will also have the possibility to add the duplicated job to the group.)
If you choose to duplicate the job and create a new group, you will be asked to define a name and a service for the duplicated job.
Then, once in the duplicated job, you will see (on the left under the job name) that a group has been created along with the duplicated job (you can view the group details by clicking on the name of the group).
If you choose to duplicate the job and create a dependency, the duplicated job will become a dependency of the job you're duplicating.