You can assign a role to a user in two ways:
Assigning a role directly to a user
To assign a role directly to a user, inside SpinPanel, choose Users > Users from the side menu to open the Users list. Inside the Users list, find the user you want to assign the role to and click the row. Inside the side pane, locate the Roles section and click the Edit button.
In the Roles side pane click the Add button to assign the role to the user. You can use the search bar to locate the role if it is not displayed.
Assigning a role indirectly through a group
To assign a role to a group, from the SpinPanel side menu choose Groups > Groups to open the Groups list. Inside the groups list, locate the group you want to assign a role to and click it's row. Inside the side pane, locate the Roles section and click the Edit button.
In the side pane you can click the Add button next to a role to assign it to a group.
For more information on creating groups and assigning users to group, you can read this article.