You are able to do it only for Confluence users, external users (resources) are excluded.
To do this, go to Restrictions option in a way as illustrated below:
In an opened prompt, click here (1) to open a drop-down menu with three elements:
No restrictions: here everyone can view an edit;
Editing restricted: viewing is enabled for anyone, however only selected and added here people are able to edit Confluence page content;
Viewing and editing restricted: only selected people can view page content and modify it.
After selecting particular position, like presented above "Edition restricted" (2), prompt view should change:
Now it's possible to find a person to whom restrictions are to be applied (1) and press "Add" button (2)
You are able to do it only for Confluence users, external users (resources) are excluded.
When all is done and set as wished, press "Apply" (3) button.