How does one join sponge development? By joining, I mean getting into the Spongepowered github organization, or getting to “actually” develop instead of only pull requests. I’m interested in helping! 
As far as I’m aware you get promoted by the current organization after showing you are able to create decent pull requests, communicate effectively with the rest of the team, and have a positive attitude towards the rest of the community.
What ryan said. You don’t apply, but staff will offer you a position.
To get staff to make that offer you should create valuable contributions to the API and/or the implementations. If your attitude and codequality are both good, then you’ll be considered.
Don’t be fooled into thinking that going through the normal OSS process of using PRs is in any way less important than being able to commit directly to the repository, PRs are just as important as contributions from core team members. We have project members who are at the “contributor” level which means they participate in our staff meetings etc. but don’t have commit access, so commit access is not the be-all and end-all of being a Sponge team member.
Basically, there is no barrier to making a contribution, anyone can open a PR. The PR will be reviewed before being pulled that’s all. Even core devs submit their work as a PR quite a lot of the time if they feel a feature needs discussion before being merged or if they want to solicit peer review.
Open source means anyone can contribute, but giving everyone commit access would make it an unmanageable free-for-all, so only core devs and leaders have that ability.
As far as “getting into” the organisation, the above comments are correct, hang around on irc and make comments on github primarily, most of the dev team don’t visit the forums anywhere near as much as we talk on IRC and via github.
But don’t proceed under the misapprehension that PRs are in any way considered “not contributing” or not “actually developing”, because PRs are the lifeblood of an open source project and are extremely important.