Answer:

You can move devices into application stacks in the 'Assets' section of the portal, as well as when viewing the 'View individual Application Stack' section.

In Assets, you can place a check mark next to the device name and select 'Manage Stacks' on the upper right corner of the table. This feature will allow you to create a new stack and move the device into that stack or move the device into an existing stack. There is also a feature to 'copy' the device into the new stack and the device will also remain in the existing stack.

In View Individual Application Stacks section, you can right select the server name in the lower left table (or highlight multiple servers and right select) and select 'Edit Stack Membership'. This will allow you to create a new stack as well as move/copy the device(s) into an existing stack.