Before any of my friends came, I was used a lot.
A friend came to visit me, allowing us to go to all sorts of places.
A third joined us, and the world truly enjoyed us regularly.
The fourth addition to our group channeled us to cooler places.
Our fifth member allowed us to move faster than any jet,
but the sixth made us a menace to those around us.
Thankfully, the seventh allowed us to relax and be entertained.
The eighth transformed us into a useful medical device.
Who am I and who are my friends? What did we become?