Some steam games allow you to create white-list based around users within a Steam Group;
If you need to grab your Steam Group 64 ID these tips below will help you out;
Firstly you will need to create a Steam Group and set the 'custom url'.
Then it will have an URL like this:
Change it to look like this:
Navigate to your new URL in your web browser, in this case we will go to https://steamcommunity.com/groups/multiplay/memberslistxml/?xml=1
You will want to grab the long number within the <groupID64> tags.
We've highlighted the number within the Image below to show what section you need to grab from your own webpage.