The sender of the emails that the candidate receives when connecting to your company is the department manager for the department the candidate connects to. 

To find out who is the department manager you can go to the three dots in the upper right corner Settings → Departments. Next to the department you will see who is the department manager. To change the department manager, open the department and choose another manager. 

If you don't have any manager for your departments the Career site manager will be the sender of your connect emails.