When you set a Default Assignee, that Team Member or Guest will be auto-assigned every new task created in that Project. You can always change your Default Assignee at anytime or on the fly when leaving a new comment
Setting a Default Assignee
Open your Project in the Toybox App
Click the Project Settings Tab in the sidebar
Go to the "Default Assignee" section and use the dropdown to select someone from your Team or Project
Once you've made your selection, click the "Save Changes" button
Every new Task created in your Project will be Auto-Assigned to the person you selected