Assuming all your sheets are contiguous, then click on the first sheet,hold down SHIFT and click on the last sheet. (If they are not contiguous you will have to hold down CTRL instead of SHIFT and click on each one). Then you can highlight the row in the visible sheet and Insert | Row - this will be inserted at that position in every sheet which is part of the group.
To ungroup the sheets right-click on one of the sheet tabs.
Hope this helps.