One thing about Realeo's +1, +2, +3, -∞: it doesn't account for two people/a group who absolutely specifically want to play with each other.
What if two/three people from another site or IRL join up and specifically want to play with each other, but don't care/don't care too much about about whoever else is in the game as long as there's enough other people to make a game?
I think there should be both a "must" /in or "group" /in type option.
If someone (made up names here) "one game /in with Hamburger and/or /in with Toby", that would mean that they want to sign up for one game, as long as at LEAST one of Hamburger or Toby would also be playing with them, but if the other one ended up in a different game or not playing, that would be ok.
Example: There is a group of four friends, Hamburger, Toby, Bob, and Alice. They all agree to play mafia. Bob wants to play with Toby and Hamburger, but doesn't like Alice's playstyle, but doesn't want to hurt her feelings and won't tell her that. Toby and Hamburger are both willing to play with all of the other three. Alice only wants to play with Hamburger, and not the other two. So, the PMs to whichever mod type gets these things are:
From Hamburger: "Group: 'Bob' 1: one game, MUST and OR or, /in with Alice, and Bob, and Toby."
From Toby: "Group: 'Bob' 1: one game, MUST and OR or, /in with Alice, and Bob, and Hamburger."
From Bob: "Group: 'Bob' 1: one game, MUST and OR or, /in with Toby and Hamburger. /out with Alice."
From Alice: "Group: 'Bob' 1: one game, MUST and OR or, /in with Hamburger. /out with Toby, and Bob."
The "Group" tag is to let the mod know that these people have discussed the game amongst themselves already, and the name "'Bob' 1" is one of the player's usernames, optionally followed by a modifier, to make it easier for the mod to look at those PMs together. (The modifier is in case a player is trying to set up multiple games with different friend groups, to keep the different groups' PMs separated.)
So, what would happen is:
Alice and Hamburger are matched, because Alice would /out with anyone else and Hamburger is fine with playing with her.
That leaves Bob and Toby, who are then matched because they are fine with playing with each other.
The two pairs of matches end up playing different games with whoever else, but all end up happy because they got to play with someone they know.
"But what if the whole group wants to play together, or someone/everyone forgets the group tag/to write /in with their preferences, or someone tries to pretend they're in a group but the people they're trying to group with don't know them?"
If the whole group wants to play together, then put them all in the same game. Or, if the logistics don't work for that, split them into as few groups as evenly as you can. So, for ten people, one group of ten is best, but if you have to split them, a group of six and four is better than three groups of three, five, and two.
Or: If someone's faking it, OR the rest of the group forgot to /in with each other, treat both those cases like this: Match up the one person who wrote /in with <a whole bunch of people> with one of them, but otherwise ignore their list as there's no evidence these people are trying to group together. It's on the person trying to form a group to make sure everyone playing does the PMs right if everyone really does want to play together. (Although, the mod *might* inform him that "No one in your list used your group tag. We can not treat you as a group. <link to info about forming groups/correct PM procedure> At best we can try to match you with one person on your list. <link to more info about that>" That doesn't tell a faker anything they didn't already know, whereas if it's a legit group it gives Bob the heads up that he needs to prod the rest about that. Or... what if Bob sends a must /in list but makes it clear they are not in a group? This means that the mod knows they don't need to send a PM about it, and can just match him with someone.)
Example: Bob PMs /in with <list of 20 people>
None of those people on the list /in with Bob or each other. Five people /out with Bob. Four people don't /in on any game, and the other six /in for one game.
Result: Match Bob with one of the six people who didn't /out with him and do want to play. Ignore the rest of Bob's /in list, and continue organizing the game list as per normal.
Tl;Dr: I at some point might want to introduce someone/other people to this site, but only if we would have the option to be *guaranteed* to be able to play in the same game together. I'm assuming that the fact that there even is an /in-vitational thread means that this is not necessarily easy to achieve through normal queuing channels, therefore I'm asking for a mechanic for it here.