Could help but notice that before a crewmate joins, they usually try to and considerably help Luffy stop a main enemy of an arc or at least defeat one of the main enemy's prominent underlings:
Zoro - defeated Morgan alongside Luffy
Nami - tied up Buggy's floating body parts
Usopp - vs. Jango, in addition to punching Kuro at one point
Sanji - vs. Pearl
Chopper - vs. Chessmarimo
Robin - helped reveal Crocodile's true identity, saved Luffy from a sand pit, and tried dowsing water on Crocodile himself
Franky - vs. Fukurou, in addition to trying to fight off Lucci and injuring Spandam
Brook - helped fight off Ryuma, Tararan, and Oars, in addition to revealing the weakness to Moria's powers is salt
Jinbe - helped fight off Wadatsumi
If similar logic could be applied to Yamato's case, then I'd mention her helping fight off Ulti, Hatcha, and fighting off Kaidou himself alongside Luffy.