When you're in the details of a group, you can click on the three dots at the end of the line of a job and select the 'Remove job from group' option.

You can remove a job from a group using the corresponding icon (broken chain) in the group details. When there is only one job left in a group, the group is deleted (but not the job of course!).
Note that when you remove a "parent job", the dependencies are also removed from the group.