Hello InstaDRY,
In order to achieve this, you have to do a bit of customization. Firstly copy your New Activity screen then you can start customizing the screen. In the stock screen the From and From name field is updated by the value of Current User logged in. You have to change it to the value whom you are assigning this activity (any staff). Once the value in the From(email) field is updating from the staff email, you can add a button object to the screen and assign actions to Retrieve value from table where you can retrieve EmailSignature from user table based on the email in From field and place it in ActionResult1.
Then, using Character Function action join the email body with the email signature placed in action result in earlier step and then place the result in ActionResult2. In the last step, Enter value into the Field on screen and update the Email Body field with ActionResult2.
So, whenever you want to update the email body field with the email signature, just click the button and it will update the field based on the email provided in from field.
There are multiple ways of achieving this, all require different method of customizations.
On of the easy option with no customization is, if you are using only one or few templates you can also create a copy of the templates with each user signature at bottom.
Need more help with customization? Ask us about Method Professional Services.
-Inder