You must create groups before you are able to assign tests to them by going to Test Settings > Groups page. Here you will have the option to add a new group or to delete existing groups.
Also, on this page you can click on the star in same row as a group to add it to the sidebar for easy access.
Adding Tests to Groups
There are two ways to add a test to a group:
Add a Test to a Group on Test Details
Find the test you want to add to a group. In the test settings bar you can select Groups dropdown. From here you can select an existing group from your list.
Bulk add tests to a group
Want to add more than one test to a group? Go to All Tests and select the tests you wish to add to a group. Select at least one test to open bulk edit options.
Once at least one test has been selected you will see the bulk edit options appear at the top of the page. One of the options will be a Groups dropdown. Click to view your list of existing groups and to choose a group to add the selected tests to.
Apply to Selectedbutton to save your bulk edits.