I have been developing a site with the Drupal 7 Organic Groups (OG) module today and I found the need to grab a bunch of users from a group depending on their group role. The first parameter here is the group GID (not the node ID) and the second is an array of role ID's to use.
$administrators = og_get_users_by_roles($group->gid, array(4));
This is fine, but as I couldn't be sure that the role would have the same ID on my production server I needed to use a function that loaded the role ID based on the name of the role I wanted. The result of which could be fed back into the above module. It turns out that OG doesn't have a function like this (that I could find) so I wrote the following.