There is currently no report that allows you to access the number of members per group.
There is however a workaround. To access the number of members per group in each site, follow these steps:
Select New Report
Select Sharepoint Group as your object type
Add a Members column
Click Next (or Save and Next if you wish to save the report)
Select your Target
Select all by clicking the
Click Export to export your data to Excel
Open the report.
Select the empty column next to Members
Insert this formula into the formula bar
Note: Remember to replace C2 with the actual call containing the members.
Drag the formula down to apply it to all rows.
You will now see the number of members in each group.