If a volunteer is getting a blank screen when trying to volunteer for a position or request a sub for a position on the Web Terminal, this means they are likely using an unsupported web browser. Internet Explorer 7 is not a supported web browser for the Web Terminal as it is older and is not able to display some of the features of the Web Terminal.
First have your volunteer verify that is the web browser they have by having them open a web page, click on the Help menu at the top of the window and select, "About Internet Explorer." This will let them know what version they have. Once you confirm it is version 7, they will want to update to Internet Explorer 8 (this is a free download from Microsoft) or higher.
Note: if a volunteer is using IE 9 and experiencing this issue, they likely have "Compatibility mode" on and will want to turn this off in the Tools menu.