You need to find their profile page (you need their profile name, and would search under "users") then you can see what Ideabooks they have created. Alternatively, you can "share" the Ideabook with them, so you will get notifications on your own page whenever they add a photo. I think you can also " follow" each other....though Houzz support can likely confirm all this!

That is true. The easiest way to get access to a friend's ideabook is to have them share it with you. To do that, they must go to the ideabook, click Collaborate, then add your email address to share.

Following each other is also a good bet. :)