There are two different kinds of properties you can create, and both can be accessible from Web Reports:
Analyses are useful if you want to be able to group properties and/or limit the scope of endpoints that report for the property. So, for instance, in this case, if you have non-Windows machines in your environment, you would likely want to leverage an analysis for this property to limit its applicability to Windows systems.
So, in the ‘Create Analysis’ dialog, you would give it a name and description, ideally select a custom site to create the analysis in (vs. the Master Action site), then in the ‘Relevance’ tab, you could limit the applicability to Windows in a few ways, but perhaps easiest by selecting the 2nd option (Computers which match the condition below), then selecting ‘OS’ contains ‘win’:
Then in the ‘Properties’ tab, you would define your property. It should look something like:
Note that smart-quotes can cause issues here when copying-pasting. Try the following:
members of local groups "Administrators"
Once created, and after Web Reports refreshes, the new property will be available in the ‘Edit Columns’ drop down: