"Group" can apply to worksheets which have been grouped together
(easily done by holding the CTRL key and clicking on another sheet
tab). To get rid of this setting, right-click on a sheet tab and
select Ungroup sheets.
It could also apply to a number of columns in the same sheet which
have been grouped together. To get rid of this setting you should
highlight the offending columns then click on Data | Group and Outline
| Ungroup.
Then save the file so that you won't have the same problem next time
you open it.
Hope this helps.
Pete