I am wondering whether there's a possible bug in the "Add Group Admins&

For reference: screencast.com/.../op1v913RaT

Please open a case with Support and attach the screenshot.

If the problem is just the designer behavior, you can try working around it by creating a Constant (Group or User/Group type) that points to your app admin group, and referencing this constant to populate the "New Administrators" input.

@Elizabeth: thanks for the suggestion, I actually hadn't tried that yet. I just did try, with constants set to "group" and then "user or group", both referencing the Admins group I need to use, but when I try to run the process it causes a failure at the smart service node: http://screencast.com/t/o61UtVc4Wc

Ok then it looks like the smart service is defined incorrectly to take a list of users only. Submit a support case ASAP if you haven't already.
It's not a good long term solution (since new admins won't get added dynamically) but for now you can use getdistinctusers(topeople(cons!ADMIN_GROUP)) to add the current group members as administrators of the new group.
The other option is to create a new smart service plugin that does the correct thing -- should be pretty straightforward using the API Call GroupService.addAdminGroups(): forum.appian.com/.../GroupService.html

I'd love to open a support case, but I don't have the necessary action available here on Forum (nor do any of the other partners I just checked with), just fyi.

Mike, I am going to email you directly about this issue. We can end this Forum thread.

The documentation, forum.appian.com/.../Add_Group_Admins_Smart_Service, has been updated as per the functionality of the Add Group Admins smart service.

Thanks again, Mike, for the feedback. The enhancement ticket number for adding groups as admins to a target group is AN-41877.