When you share a card, you can assign a role to the new member of the card. Since members can view shared child cards their permissions can also be inherited. There are six different roles someone can have on a card:
Author
The original creator of a card. This cannot be changed.
Always retain the card even if it is cut from a parent (i.e. all of an author's cards are automatically 'kept').
Assign all user roles, and promote/demote moderators
Cannot be demoted
You will know if you are the original author of a card as the edit button will display a pencil icon.
Moderator
The reviewer and curator of the members and child cards.
Change the Default Joining Role and Default Role in Parent
Cut child cards
Promote/demote members (except Moderators)
Editor
The editor of the current card. Cannot change member roles.
Edit the card content
Manage comments (incl. deleting other users)
Shared Tag management
Contributor
The best way to collaborate with someone, allowing them to add cards but not edit your content.
Sharing new child cards within this parent card
Cutting cards they added within this parent card
Reader
Share your cards with others without worry of any additions or changes. Things they can do:
Liking
Personal tag management
Adding this card to another parent
Adding a child card as private (not able to share with others)
Leave (the card)
You can also "Remove Member" from the card if you would like to revoke their access. The edit button will have a different icon depending on you role on the card.