Custom Access to Pages

I’m trying to set up access to a booking page for volunteers. I would like to give users access to 1 specific page where they can plug in their availability on a calendar. I’ve looked over roles, capability, and admin in the codex and am not sure what route to take. I need volunteers to have read and write access to that page without being able to change the sites other pages (no access to the site’s admin).
I’ve got the volunteer calendar set up as a plugin placed onto a wordpress Page. Should I set up a custom role for the volunteers, create a custom WP login, mod the default login, or password lock the calendar page?

I’m running this on a local dev site so I can’t provide the URL.