You can set users to be default assignees for specific observations. Default assignees can be limited to some specific place, category or place + category combination. You must limit the default assignments by selecting at least one category or place (red help text on the picture below). However, you can add all places and/or categories to that list if needed.
The default assignee is edited from the edit user page:
From the left menu click on Administration --> Users --> select the user --> scroll down to Default assignee section
You can also find the similar settings for users by using a role from the role edit page:
From the left menu click on Administration --> Roles --> select Role name and click 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.