Here you can add or remove users to/from the currently edited role. These users will be authorized to perform actions according to the permissions granted to the role on the Permissions tab.
If you wish to add users, click the Add users button and check the boxes next to the appropriate users in the displayed selection dialog. Only users who are assigned to the same site as the role can be chosen (global roles may be assigned to all users in the system). You can enter a name or its part into the textbox above the list and click Search to quickly find any user. You can use the Valid to field at the bottom of the dialog to assign users to the role for a limited time only. The Calendar button can be clicked to easily select the exact date and time when the role should expire for the user. If this field is left empty, the users will be assigned to the role for an unlimited time period. Click OK to apply any changes.
The Change validity action that is available for every listed user may be used to prolong or shorten the time interval for which the user should be assigned to the role. This way you can set an expiration date or reactivate expired roles for users.
Users can be removed from the role at any time using the checkboxes in the list together with the Remove selected button.
More resources on Roles can be found in Developer's Guide -> Development -> Membership, permissions and security -> Security model overview.