1. Get the URL of your steam group (navigate to your group on steam, right-click your mouse and select "Copy Page URL")
   
   It will look something like:
   https://steamcommunity.com/groups/pingperfect
   
   
2. Then add the following to the end of the URL:
   /memberslistxml/?xml=1
   
   Which will give you something like the following:
   https://steamcommunity.com/groups/pingperfect/memberslistxml/?xml=1
   
   
3. Visit the URL created in step 2, it will show you XML code, at the top of which you will find your Steam groups ID.
   For example:
   
   
   <memberList>
<groupID64>103582791433737423</groupID64>