Greetings Robert,

Email templates come out of the box with the "Dynamic Value Builder" which you can use to generate the placeholders.

When you refer to the user who is in charge or has generates the sales order you are usually referring to the Salesperson which can be found in the Dynamic Value Builder

This is the placeholder for accessing the data on the user responsible for the sale ${object.user_id}

Here you can just append signature ${object.user_id.signature} and get the signature stored on the res.user obejct (Settings->Users)

If you would like to include more information about the user like email and other things you would have to make a link towards the partner in your system that is linked to the res.user object by using ${object.user_id.partner_id}

This will point towards the partner and all the informations are available by appending "email, phone" and whatever is defined on the res.partner object

Hope this helps!