For more about Grouping, See the help article: Group, Rename, Categorize

To un-group, you can drag an account out of the group and under its respective tax line to break it out individually. Here is a video showing how to ungroup by

For more about the drag and drop feature, see the help article how to drag and drop.

Groups can be deleted altogether using the trash can to the right of the tax category drop down. You might need to slide the bar between the TB and JE panels to the right to reveal it.