Hi Anitha,
You can form groups in tableau manually or by selecting items from the visual.
By selecting on the visual ->
Select the group of items then click on the group icon.
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Now Edit the groups and items within them.
Now place the group in the visual, and also the items in the group on the shelf.
From field ->
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Now go to analytics pane and drag the totals and place in sub-totals followed by grandtotals.
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Hope it helps.