I guess you are using the Reverse mapping approach. Please, take a look at this knowledge base article
to get familiar with the process. Note the last step described - you have to enable the inverse collection option in order to get a list of Users in the Role class. After doing this, you will not even have to execute a query at all - each Role will have the collection with the related Users and you can reach it by using the Role.Users property.
the Telerik team