You can set users to be default assignee for specific observation and it can be also limited to some specific place, category or place + category combination.
The default assignee is edited from the edit user page:
From the left menu click on Administration --> User --> select User --> scroll down to Default assignee section
You can also find the similar settings for users using a role from the role edit page
From the left menu click on Administration --> Roles --> select Role name and click pen- icon to edit:
When a default assignee is set for some observation category + place combination, the assignee field will by default contain that user in the reporting form.
If the user has rights to edit the assignee they may remove the default assignee in the reporting form.