When you find something interesting in a recording you can also leave a note in the session. However, if you want to remember specific users and filter them later you can simply 'favorite' them. Here's how:
How can I find my favorite users?
You can access the favorite users from the recordings tab by adding a filter. If you need to, you can save this as a segment for future speedy access. From the dashboard go to:
Recordings > Create filter > Favorite > (Yes) Save.
If you need to have more details about the users, it may be useful to combine the "favorite" filter with the "notes" filter.