Custom capability – edit_others_posts at same level

Hi all, I’ve been searching forever on this and have finally decided to just ask and hope for an answer. What I want is a capability that allows same-role users to edit each others posts (e.g. authors can edit other authors, without being able to edit admins). It seems like this was an old feature of WP that was removed a long time ago.

I think I should be able to do this for a custom capability, but everything I’ve found shows me how to add a custom capability, but not where/how to actually implement it. Can anyone provide any direction on where to go from here?

Thanks,
WJ