When you create such list this is a special kind of mailing list - it is dynamic and its counter is 0 by design. When you add a new user to Sitefinity it will be added automatically to the list, even though you won't see the counter increase. This is by design and currently there is no way to change this behaviour. Apologies for the inconvenience.
All the best,
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items