As tickets in 7.5 aren't assigned to groups but to service queues, you'll first need to get the information about the service queue. Two possible ways to do this: via ticket info or by "Get All Service Queues" component. Once you have the queue, you can iterate through the security groups assigned to that queue, get all users in each group, and add the users to your drop down variable.
So, if you are working with an IM and have the session id, you can use "Get Incident Ticket By SessionId" to get the IM ticket info. This variable will contain a property of "Currently Assigned Queue" and that property will have a property that is an array of security groups mapped to that queue. Obviously, this is only useful if you are wanting the user info for the currently assigned queue, so it may not be what you are after.
The second option will definitely get you what you want, but it may take longer depending on how many queues you've got. You can use "Get All Service Queues" to get an array of queues which, again, will have a property of security groups. Iterate through those groups and either "Get Users By Group ID" or "Get Users In Group". "Get Users By Group ID" will provide much more info for each user, so it would likely eliminate any need for a call for additional user info.
Third option that I haven't mentioned yet. A custom component to pull directly from the SQL DB. You'd need to work with "ImServiceQueue", "ServiceQueueGroupMap", "Group", "User", and "UserGroup", I'm guessing. But you could make it do exactly what you need in a single component.