As a Salesforce Administrator, if you need to setup Email Signature for a new Salesforce user, how can you do this? You are not going to ask for their Username and Password and login on their behalf. This approach is not good and no one likes to share their password with you.
In Salesforce, an Administrator can login as any user. To do this, enable this feature under Setup > Administrator Setup > Security Controls > Login Access Policies.
This feature is not available out-of-box. You will need to request it from Salesforce support.
Once this option is enabled, in the Manage User page, we will see that there is an additional ‘Login’ button next to each user. Of course, it does not apply to certain profile and conditions.