At the stage level, hover over any of the circles or the stage row to show you the plus icon.  

Click on the plus icon to open the sidebar. The sidebar shows you the roles and staff, collectively known as the ‘Current team’ assigned to this stage. 

In the sidebar, under 'Add roles', click on a role to assign it to the team. This lists all the roles you have in your system. When you assign a role to the stage, the role will be allocated at 100%. The 100% is the effective utilisation of the combined members of the team. See Clicking on staff member, to change the number of automatically allocated hours.

Click out of the sidebar to see the changes.

You can add more roles to this stage, as required by repeating the steps.