For Example.
Group 1
---------
1
2
3
4
Group 2
---------
1
2
3
4
5
Group 3
----------
1
2
3
Go to Insert running total field
There choose new and give any name for the running total fields
Then in the fields to summarize option choose any field and in type to summarize choose count
In evaluate option choose For each record
In reset option choose
On change of group
Then click ok and insert it in your report.