You need to be on a Sharer plan for this.
If you have any problems setting it up, then give O2 a call on any number in this guide.
Advice from an O2 Guru regarding a way to get through:
If you call 202, when it comes to the reason you are calling stay silent for 15 seconds and you will be taken to the keypad options.
Press 2 for Everything Else
Press 5 for More Options
Press 7 for Anything Else which will put you though to an advisor.
Press Upgrade, Lost/Stolen if you have to.
Veritas Numquam Perit
Until you get your tariffs changed over to sharer plans you can turn on your personal hotspot to allow others to tether from you if they are nearby.