I guess most of them, aside from Hisoka, have gone through a lot together. Considering their profession I don't think there should be any unconditional trust between them, but probably they believe they can judge each other and the situation somewhat accurately, again likely with the exception of Hisoka. They are all powerful but they should know there are people, like assassins of the Zoldyck family, who could take them out if they were all alone out there. Also, they can perform far greater heists together due to simple logistics. So, it's a risk to trust a technique like that, but it was a risk worth accepting right then and there.