You can get a list up of all the Users in a particular group by creating the filter on your User Object. You can then filter it by Role, using a criteria of: "Roles.Role Is Equal To (<<ROLENAME>>)"
The bit that will be difficult is to only see users that have a request assigned as there is no link from the User to the Request Assignment Object. You might be able to create some form of calculation on the user object that will update with the amount of request assignment and then use this as a criteria in your query / filter. However this is not something that I have tried and is just from the top of my head.