It's not complicated. You can HIDE or UNHIDE anyone as often as you want to. It makes no difference for anything except how you can see their posts on your screen.
When you have a member on HIDE, the fact that they made a post still shows up, but the content of their posts is all hidden. Try it with this post right now. Click HIDE above these words, and you'll see that all of my posts will be invisible as to their content, but their location will still be seen. Then you can UNHIDE them just as easily by clicking the green VIEW button, and you're right back where you started, with no consequences.