How can I sort mentions into groups?

You can find the Group button below each mention.

You can use this button to add a particular mention to a previously created group or you can create a new group. You can check the content of a group in the filter menu, located to the right of your project dashboard in the Filter by groups section.

You just need to select a group and the project will show all mentions within a given group. You can also add new groups or edit existing ones here.