If you don't see any option to send a text, you may not be an admin on the account. If you are, you may not be on the latest version of the app. Finally, you may not have added the feature yet. You can do that online, but not directly within the app.
Text messaging was just added to the most recent version of the iPhone app and is only visible to users signed in as an admin. Make sure you're on the latest version (2.3.3 or above) and that your username is one of the admins for your Grasshopper account.
If you have the latest version and you're signed in as an admin, make sure that you have push notifications enabled for the Grasshopper app. Text message notifications won't be delivered w/o the push notification.