Navigate to the activity that you wish to restrict access to

Scroll down to the 'Restrict access' category and click to expand

You'll be presented with a list of options for restriction. In this instance, select 'Group'

Allow only students who belong to a specified group, or all groups.

Pay close attention to the Eye symbol next to the group that you select. If the eye is open, as pictured below, then students who match the selected group will be able to see the activity

Clicking on the eye changes from open to closed. When closed, like shown below, the students within the selected group will not be able to see or access the quiz.

Hidden entirely if student doesn't meet this condition

Restrict Access by Groups