Good idea for the FaQ topic, I'll probably make an official one later. ^^
Who becomes a crew member is based on a lot of things: how long they've been in the guild, their overall attitude, manners, and grammar, and contributions to the guild, among other things. By contributions I don't mean donations as much as I mean things like posting up stories, making guides, etc.
A one-shot is a one-chapter story that is usually under a certain amount of words, usually, say, under six- or seven-thousand. Not that these guidelines are set in stone, but that's just how most are. Many one-shots are written about things like relationships, whether it be family fluff, romance between two(or more >.>; wink characters, or full-out smut(which is NOT allowed here in the guild. Or on Gaia, for that matter.)
A short story usually has under, maybe a dozen chapters and spans over a longer period of time than a one-shot. These can be a small adventure, relationships between characters, or a wide variety of other things. Again, not set in stone. Feel free to define these however you like.