Trask
Trask must be a member of your party before you can leave your quarters in the Command Module of the Endar Spire; however, he's removed from it before you enter the Starboard Section.
Carth
Carth must be a member of your party before you leave your hideout for the first time on Taris.
Mission
Mission can join your party when you encounter her outside the village gate in the Undercity on Taris; she must be a member of your party to rescue Zaalbar from the Gamorrean compound in the lower sewers, and subsequently to lower the force field preventing access to the upper sewers.
Zaalbar
Zaalbar joins your party when you rescue him from the Gamorrean compound in the lower sewers on Taris, which is a prerequisite for Mission to lower the force field preventing access to the upper sewers.
Bastila
Bastila joins your party after completing the search for her on Taris and returning to your hideout.
T3-M4
T3-M4 joins your party after purchasing him from Droids by Janice in the Upper City North on Taris; T3-M4 must be a member of your party to open the security door to the Sith base to allow you to escape the planet.
Canderous
Canderous must be a member of your party when he takes you from Javyar's cantina in the Lower City to Davik's estate when escaping Taris.
Juhani
Juhani joins your party on the Ebon Hawk after activating the quest for the Star Forge on Dantooine if she wasn't slain by you during your Jedi trials.
HK-47
HK-47 joins your party after purchasing him from Yuka Laka in the droid shop of Anchorhead on Tatooine; this isn't necessary unless you want a peaceful resolution to the conflict with the Sand People.
Jolee
Jolee joins your party after completing his request in the upper Shadowlands on Kashyyyk; he must be in your party to lower the forcefield preventing access to the lower Shadowlands.