Once you have created the role and define the various permissions per object according to the function of that role, you can now assign the newly-created role to a existing user.
To do this, do the following:
On the top navigation bar, click on your organization menu
Click on Manage Users tab
On the User List, hover your mouse pointer over the desired user’s email, to reveal the hidden icons.
Click on edit button to edit the user account, the edit user page will open up.
Under Roles(Permissions), select the desired role.
Click on Update Account to save the changes.
When a person has more than 1 roles, their permissions are an aggregation of the permissions from all roles, so one role could allow them to manage groups, another one would enable them to manage coupons e.t.c