Here's actually the full start of the log from the very first line:
[23:29:39] *** Joins: sk89q (~firstname.lastname@example.org)
[23:29:39] *** anarchy.esper.net sets mode: +nt
[23:29:40] *** sk89q sets mode: +s
[23:30:35] *** Joins: luacs1998 (~email@example.com)
[23:30:57] * luacs1998 shall take the debate position of the forge supremacist
[23:31:03] *** Joins: TabletCube (~TCube@22.214.171.124)
[23:31:11] <sk89q> lel
[23:31:11] <sk89q> hi
[23:31:14] *** Joins: Dark_Arc (~Dark_Arc@126.96.36.199)
[23:31:25] <Dark_Arc> oh hai
[23:31:30] *** Joins: blood (~firstname.lastname@example.org)
[23:31:34] *** Joins: Johnson (~Johnson@24-178-233-39.dhcp.stls.mo.charter.com)
[23:31:39] * luacs1998 shall take the debate position of the forge supremacist
[23:31:47] <luacs1998> :P
[23:31:51] <blood> good name sk89q
[23:31:52] <blood> =)
[23:32:07] <luacs1998> heh, that steve jobs cube computer
[23:32:12] <sk89q> haha
[23:32:15] <sk89q> I should probably op some people
[23:32:17] <sk89q> I am pretty bad at that
[23:32:24] <sk89q> #sk89q has like the same 6 ops for 3 years
[23:32:44] *** Joins: cindylo|laptop (~~@aero-tech.me)
[23:33:11] *** Joins: Dpa1991 (~Dpa1991@pool-71-100-156-90.tampfl.dsl-w.verizon.net)
[23:33:13] <luacs1998> lol
[23:33:19] <Dark_Arc> what's that have to do w/ anything?
[23:33:19] <Dark_Arc> lol
[23:33:47] <sk89q> appears blood is not registered
[23:34:07] <luacs1998> sk, have you /cs register 'ed this channel?
[23:34:09] <sk89q> yes
[23:35:29] *** Joins: AbrarSyed (~AbrarSyed@abrarsyed.me)
Frankly, Sponge wasn't really started in an organized matter at all. I created #nextstep and then advertised it in #sk89q and #cauldron.
People came in... a lot of people. Something like 700-900 people in the channel.
Then I made this document. This document was the most important document of #nextstep. I actually gave pretty much everyone that asked access to edit it.
Everything was essentially via consensus as I asked questions, took opinions, and then put them on the document. Eventually there was a lot of support for a new project, and then people asked me to be leader of this new project. I personally asked blood to help as co-leader, and both of us created the IRC channels and registered the appropriate social media accounts after people seemed to like the name 'Sponge'.
At this point, I invited all the devs of the major projects (I'm talking like literally everyone -- people from Canary, Spigot, Sponge, Bukkit, Forge, Essentials, etc.) into a private developer channel. At this point, no one had officially signed up to work on Sponge... within the next few weeks, people left and stayed, and we ended up with people from Forge, Spout, Bukkit, Essentials, and some other projects staying on.
tl;dr Sponge was created by a mob and put blood and I as leader. A few people from some of the major projects became committed to the project through the course of the next few weeks after I personally invited them all into Sponge's private developer channel.
I also still have all the IRC logs from the time, including from all private channels, if you have a question.