Step 1: Click on the Groups icon.
Step 2: If you have not already, add the desired users to the project and create the desired group. Then click on the users you wish to add to the new group. When the users are selected, a teal highlight will appear around their names.
Step 3: When all of the desired users have been selected, click on the "Add User" icon next to the group you wish to add the users.
Please note that you cannot add project admins to groups because they already have access to all of the project files.
You can also add users to groups from a files' viewer list.