Assigning and managing users on a lead
In the back-end menu, go to CRM › Leads
Select the desired customer and go to Users Assigned.

To assign a user click on add user or click on 2 dots on the right to

A window will pop up and it will show all the users that you have and you can choose one or more to assign to them, once you select who you want to assign to, click on the blue Update button.