Chakra, actually. As seen with how much it strained Obito to search through the dimensions for Sasuke with his Kamui, the cost of summoning several people and all 9 bijuu from another dimension would be even more colossal. Additionally, considering bijuu were given to the other villages in the past, could be the kage had some means of summoning them akin to Madara summoning Kurama, that could have had its part to play too.
Assuming that that is what the Kage were needed for, how in the world did they get Kakashi and Sakura too? I can see Naruto and Sasuke because the sage gave them some of his power, but Kakashi and Sakura didn't exactly have contracts on them in almost any sense. Heck, Kakashi getting the sharingan back was deus ex machina in a way.
Considering that Edo's have been recognized to have limitless stamina, just having Tobirama, Hashriama, and Hiruzen should have been enough chakra to bring the beastss and the humans back.
Come to think of it, The SoSP probably had the ability to summon all 9 beasts, and just not the chakra reserves to do it. Not to mention he had the ability to finish sealing the Kaguya despite being in a different dimension and all...
Oh, and fun fact, this is one old toad:
http://naruto.wikia.com/wiki/GamamaruJust realized that he was around when the SoSP was. That's like, unfathomable for me.