I haven't quite figured out how Dodo became a crew member. There was an Anne Chaplet menioned in "The Massacre of St Bartholomew's Eve", but that was or may have been Dodo Chaplet's great-great grandmother and she may or may not have been played by Jackie Lane. Dodo comes aboard the TARDIS in London in some future time by mistaking it for a real Police Box and coming in to report a crime!. This doesn't seem to have occurred in any BBC episode that I know of, it may have occurred in one of the Dr Who books. If I find out later, I'll post it here. In "The War Machines" she runs up against an evil computer that controls her mind and wipes out memories of all her travels with the Doctor and she just wanders away
Dodo Chaplet was played by Jackie Lane (born 10th July 1947 in Manchester). She was only 18 when she checked in as Dodo in "The Massacre of St Bartholomew's Eve" which makes her the youngest female companion, and played her until "The War Machines." After her Dr Who stint, she became an agent and represented both Tom Baker and Janet Fielding (Tegan Jovanka).