SMS is a messaging system inextricably linked to the cellular radio network. If you wanted to be to use SMS on a WiFi only tablet you would need either to use an Internet to SMS gateway or have a pair of cooperating apps one on the phone and one on the tablet. Such a solution exists for being able to send SMS from a PC with Chrome installed by using a Chrome extension and a cooperating app for the phone (deskSMS?)
On the other hand there are plenty of instant message apps that run natively on phones, tablets and PCs. Which one you go for may depend on what your friends have but there is a service built into Google+, there is Google talk, Facebook has an IM service, there's WhatsApp and a good many others. Beware Apple and Blackberry have proprietary instant message apps that try to get other people onto their platform. Best to encourage friends to use an open standard or at least well ported service.