When you're browsing Mentions center, you can find Group tab at the bottom of each one of them
When you click on the tab, you'll be able to add specific mention into group which you've created before or the one you can set up right away, for example:
So basically you can group your mentions however you want - by positive or negative feedback, posts coming from your brand ambassadors, mentions you'd like to show in your pdf report or add to widget.
If you need to add more mentions to one group while scrolling through your results, you can tick squares in the corners of mentions
and then choose the group at the bottom in global settings
You can easily manage your groups on the right side of Mentions center, under Filters section. By clicking on the chosen group, your results will be filtered for the mentions you've assigned there before.
Here's a video tutorial on how to create a group, step-by-step:
If you want to learn more about groups, Here's a more in-depth guide to using Groups - including how to set up e-mail and mobile notifications for individual Groups.