Gets a group by slug.
The getGroupBySlug() function returns a Promise that resolves to a group whose slug matches the given slug.
The slug is the end of a group's URL that refers to a specific group. For example, if a group's URL is 'https:/example.com/groups/{my-fitness-group}', the slug is 'my-fitness-group'. The slug is case-sensitive. It is generally based on the group name, but for secret groups it is
an autogenerated string of characters, for example, 'https:/example.com/groups/{5D3yTX}'.
Note: For SECRET groups, only site admins, group admins, and group members can see a group and its content. However, if the suppressAuth option is set to true, all permissions are overwritten, and all site members (including non-group members) can see a group and its content.
Part of a group's URL, for example, 'https:/example.com/groups/{my-group-slug}'. Generally based on the group name, but for secret groups it is
an autogenerated string of characters, for example, 'https:/example.com/groups/{5D3yTX}'. It is case-sensitive.
Authorization options.