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.
Unique part of the group's URL, for example group-1 in https:/example.com/groups/group-1. Pass only the slug. Case-sensitive.