How to begin.
Starting a game can be hard. You want the players to have the freedom to make whatever they want. Too often this ends up with a tavern, in some town somewhere, that has five rugged strong silent types, and one wizard. And then you are forced to engineer some set of circumstance to motivate them to interact.
Granted, what I described is a sort of stereotype of gaming. But it happens. The question is, how do we prevent it. We want the characters to be unique (most of the time) and the players to have agency when they create them. But they will ultimately need to mesh, unless you are specifically hunting for a game that prioritizes a lot of PC to PC conflict.
You can dictate the types of characters, races, classes, enforce requirements for certain background elements, establish some sort of structure that all the PCs are placed into, and then work with the players to come up with characters and backgrounds that mesh with the story and world you have set up. It's like railroading except there are choices. And you're not dictating the story, you're establishing a framework for players to use when building characters.
Examples might be a game where all the PCs are part of a military unit, or governmental agency. Maybe your game world has no races other than humans. Maybe you're doing an Elf only game (but why?). You get the drift though. There are reasons why the characters need to fit into certain fairly open parameters.
When you do something like this, make sure the players know in advance. If anyone wants to be something outside the established framework, get their reasons. See if there is a way that it can be worked into the framework. In a recent game one of the parameters was that all of the PCs were from a specific country, or in service to the country. One of the players wanted to be an Elf (they have their own country) so we decided that he was in the army of the game country, and had been for some time as part of an officer exchange sort of thing. Granted, the Elves gave us someone, but didn't want anyone in return. They are a bit xenophobic.
As long as everyone knows and understands why you want this framework, there shouldn't be any issues. The players will have some level of common ground and stand a better chance of surviving the initial 'getting to know and dislike you' phase of the game.
In Medias Ras.
In the midst of things (in English) is another way you can start. Don't start with how we got together as a team, let's jump into some action, moving forward through the story from a point after the beginning. Then do flashbacks.
Flashbacks are great in TV, unless you overdue them. I'm looking at you CW! There are a lot of ways you can 'trigger' a flashback in an adventure. I recommend not doing it in the middle of a battle, but in moments of tension they can be fun. Freeze the scene, then ask the player who is currently in the spotlight and about to do something crucial a question. "When was the last time you did this with the group that it went poorly?" Have them set the scene. The focal point is that the same skill arability is the center point of the flashback. Once they set the scene, let everyone chip in a bit of how that situation played out. We're trying to build a shared history that defines the relationships that we have artificially created because we began in medias ras. Once the scene of this previous failure has been described and everyone sort of has an idea how it went, come back to the 'present' and then pick one player who is not the person performing the action and tell them, "This reminds you of that time [we just described] what, if anything, would you say." This let's you create the past and it can also help the PCs relate to one another. Does this PC say something encouraging, or do they remind the person about their failure, or do they say nothing and only mention it after the situation is resolved.
We're all from the same village/neighborhood/complex. Established common ground, maybe the sole survivors of a tragedy (how many 'sole survivors' can there be in one place?). They all know each other to some degree, even if it's just by reputation. Then go ahead and have them all meet in the same tavern, it's not far from their homes after all.
Freedom is great, but make sure that they know enough about the world and your skill as a GM that they aren't going way out of your comfort zone when they build a character. Five strong silent types who mind their own business make for a lousy adventuring party (unless that's what you need).
Don't be afraid to say, "That just doesn't fit, but....." and offer something as an alternative. If your group is a good one they should totally understand the need for some structure. You're not oppressing their creative freedom. You're trying to run a game that everyone, even you, gets to enjoy.
Do you have any favorite way of getting the PCs to become a group? Share them in the comments below. I'd love to know what you think.