To add a new role, click on the “Add new role” icon.

In the role definition window you can set the role’s name, select a convenient icon and assign SharePoint users and groups. To do the latter, click the “Edit” button or either the “Included” or “Excluded” areas.

To define who is to be included in the role and who should be excluded from it, we use the “Change the membership” window. By default, all users are included in the role (“Assign all users to role” is selected). If we want to assign only specific users, we should use the search box, enter a part of user name and click the “Find” icon. Found users will be shown on the list below. To include user in the role, click the green icon. To specifically exclude user from the role, click the red icon.
The exclusion option is useful when we want to include in role a group, but without specific users.

Note: You can select the user named “Author of current item”. This user is dynamically assigned when the workflow starts – it is the user that has created the item on which the current workflow instance is running on.