On adding a user to the call group the following error displayed in Lync 2013 client:
One or more selected contacts cannot be added as a delegate or team-call group member. Either the contact is outside of your organization, is an automated service, or is not running your version of Lync.
By design of the Lync application it is not possible to add a user to the team-call group if the user has never signed in with their Lync client. Until a user signs up to Lync client, Lync cannot determine their client version and returns the above-shown error.
Ask the user to sign in with their Lync client, after that try to add the user again.