The admins of a company can manage email and text message templates. You can find message templates in settings -> message templates.
Email templates are added by choosing add a new email template in the settings. Email templates are available to all users on the invitation sending view, and the templates can also be modified once they have been added. Any modifications you make will not affect the existing templates.
Add text message templates by clicking add a new SMS template. Text messages are so short they are not given a name, unlike email templates.