For those that have taken the atDNA test at Ancestry.com and not gotten any Circles or maybe you have Circles but none for the ancestor you are wanting to research. When you are viewing documents that are attached to your ancestor in others trees look at the right hand column when viewing the document or picture. You will see the button that tells you who the doc or pic is linked to. Other ancestors or persons in or mentioned in the picture may be linked. Below that is a Saved By listing for other people who have saved that picture or document which should mean they are interested or related to the family in some way.
Click on any of the Users. This should take you to their profile page. From there you should be able to toggle thru any kits you admin and check against the profile. You do this by clicking on the Hyperlinked word "your". See the arrow pointing to it in the image below(Edited for privacy). This doesn't always work and I think it is due to settings for the other profile if they privacy settings pretty tight.
If any of the kits you admin match the main profile or any of the kit's for which that profile is the admin, those kits will show below the Main profile panel. It is important to note that Absence of a match doesn't mean that they tested and do not match it. It means there is no DNA available for that profile that matches yours. They may have tested and not match...or they may not have a test at all. When you do find a match though you can then click on it to go the Match page and look at cM amounts, projected relationship, tree, etc.