I would check if your account has write permissions for the app that the dashboard is located in (Search and Reporting, etc..). You need write to be able to change permissions.
Also take a look at the dashboard's permissions (you can do this from the dashboards tab). Ensure that the group you are trying to share with is enabled with at least read permissions. You will want to display the permissions for App in the edit permissions window. You need write to be able to change permissions.