In the Get User List component, first leave the Context tab with its default settings (Get Settings from the Default Properties, Pick, and Entire Domain).
Then on the Input tab, choose "Set of Fields". What you're specifying on this tab is one or more filter criteria. So in your case, you want to filter by group membership. But, notice how you don't see that listed as one of your available choices? That's where the Additional Attributes field comes in handy. There, you can specify
any AD attribute (even custom ones) that you want to search on.
I looked it up, and the name of the attribute you are looking for is "memberOf" (without quotes, of course, and not sure if it needs to be case-sensitive but I've typed it exactly as it should be). It's looking for the DN of the group (I know this because I used
ADSIEdit to see what an example value looks like). So, you can pass it the group's DN there, as in the screenshot below. This configuration worked correctly for me. You could later get more details about a given user via Get User Details.
Two sidenotes: (1) you could, of course, use additional filter criteria in addition to "memberOf"--just specify those as well, and if you want the search to use ALL of the criteria (not just ANY of them), uncheck "Match Any Criteria". (2) If you can't or don't want to hard-code the group DN, then you can use a process variable of datatype CustomAttributeList. Just set that (and change the value dynamically), e.g. using single or multiple value mapping components.