In each project you can create roles, which allow you to easily assign tasks to a group of people or mention them in a comment.
Add a new role
To create a role, go to the people section of your project and switch from the People tab to the Roles tab:
Then, you can use the 'Add Role' button on the right to create a new role.
You'll see a screen where you can choose a name for that role (bear in mind that the shorter and simpler it is, the easier it will be to '@mention' later) and add a description so you can add any notes to define it further.
Then, choose from the people on the project by selecting the checkbox next to their name, and click 'Add Role' to create it.
Editing a role
Once you have some roles added to your project, you'll be able to see them listed in the roles tab under the people section of the project.
To the right of the role name, you'll see icons to edit (change the role details using the same screen you used to create the role) or delete the role.
Hovering over the profile image for each person currently listed in that role will give you the option to remove them, or go directly to their profile.
Clicking the plus icon to the bottom right of the list of people will give you the option to quickly select new people to add to the role.