My Sim is a Rock Star, and can hold autograph sessions by clicking on the cinema. I tried this for the first time just now, and the game told me to try and get as many autographs signed as I could within 4 hours. The problem is I have no option to sign an autograph - not by clicking on the cinema, my sim or another sim, and she doesn't have anything like autograph papers in her inventory.

Has anyone else already done this and knows how I'm meant to do it?

A bunch of sims should start to arrive and gather around you, each of which should have a conversation option of "Sign Autograph". You basically need to just choose each of them one a time in approximate order that they arrived.
Sims already at the cinema won't have the conversion option initially, but may get it later.

You prolly have to talk to ppl for awhile before the option is there in conversation. Sometimes things are released that way. An autograph though is supposed to be without damn near having to make a new friend in life so *shruggs*

Hope I helped.

That's strange, a huge group of sims gathered around mine but absolutely none of them had the option to autograph sign. I also tried to talk to them - I tried with people my sim had never met before, people they were friends with, and also her wife. Maybe it just glitched and next time it'll work :/