i managed 2 community art packs, one was of MLP Royals, the other was Halloween. Essentially its just commissioning people for their full regular prices, and having them each do something around a certain theme, then asking them not to post until the pack is released. Also then telling them that when it is released, they get a cut of the income (that is if it isnt strictly free) which acts as a specific extra thank you, and sort of acts like payment since theyre promoting the pack to their audience once posted publicly. Then after the first week or so, i assumed about 90% of all the possible income was in, so i divided the money up equally between everyone as a bonus, including myself since i also drew as well, then i just also took the last couple of $s it earned after that.
Funny enough, both times i ran into complications where my internet cut out for days and upwards of a month, and i had to use my shitty phone at a public wi-fi to message people, all the while panicking that i wouldnt be able to even put the pack up for Halloween if my internet was still out, fortunately all was well.
Which is also another point, if you have a specific day in mind, or have made date specific promotional material, make sure the artists have enough time to actually do their piece