I already knew they accompanied her on engagements, and that they also handle quite a bit of her correspondence. I did find this on Historyplex -
The current ladies-in-waiting of the Queen Elizabeth II work part-time for two weeks and take four weeks off. When on duty, they stay with the queen. They accompany her when the Duke of Edinburgh is not available. They carry bouquets that are given to the queen on her visits. They handle her correspondence. They keep track of her engagements and help her. The ladies-in-waiting attend public and personal matters of the queen.
✦ They are not paid, but given allowances. They do not receive a regular salary as they serve for honor.
✦ They serve for several years and do not retire. If a lady is unable to carry out her duties due to ill health, she is appointed as an extra, but keeps the title.
I would say it's a pretty fair job description, except that bit about serving for several years. Lady Susan Hussey has been with HM for quite awhile - since Andrew was born. HM lent her out to help Diana settle in.
Lady Susan and Anne Beckwith-Smith were so good with her before it all went off the tracks, I admired them both very much.
I was trying to look up information for when Lady Susan (now Baroness) joined the court and it was very hard to find. She's very discrete, there is very little information on the interwebs about her. Apparently she joined QEII in 1960 making this her 60th Anniversary year! Amazing woman and what an asset to the Queen.