what specific capabilities are needed for a role to be able to edit a page using Cornerstone?

I added 2 free plugins to my WP site, PublishPress Permissions and PublishPress Capabilities and set two people up as ‘subscribers’ to give them access to one page only of my website.

It works fine but they can’t edit in Cornerstone (my theme), only by using the WP editor which means they can’t see the page until they preview it - is there any way they can edit in Cornerstone – could you tell me what specific capabilities are needed for a role to be able to edit a page using Cornerstone?