The ways I figured a character would have an easy way of joining the group are the following:
1) Involvement with the current plot
2) Involvement with some group previously allied with the characters, or involved in some group working against the characters
3) Knows some character in the group from the past (family member? old friend? childhood rival?)
4) Just learned some information the group needs
5) Similar motives (shared enemy, shared goal, etc)
6) Has some skill or item the group needs badly and is looking for (example: a healer in a group that gets wounded often)
Let's presume, for the sake of simplicity and based on the character so far, that none of the six are true; the character has no connections at all, no family, no old friends, and no involvement with the plot, and, while we are looking for a healer, he wasn't one in any way or form. The character is, however, open minded, enough, I think, to accept hire or follow the group around for curiousity's sake, though not for more then a few days without a reason. It's possible the GM and/or the player could be convinced to come up with one of the six retroactively.
I did make an attempt at explaining the current plot and characters to the player, though I think it's safe to say I did not succeed in explaining it well enough that the player felt comfortable in making a character already integrated with the story so far.
As I am another player, not the GM of the group, and also as the group's 'leader' as it were, I found myself completely baffled as to how to include a new player character in-game, and clearly both the player and the DM were just as confused as to how to involved a new character. Any suggestions from this community would be gratefully considered (and I can and will talk to the GM or player about them if it requires their involvement. I can't imagine a solution which does not in some way involve the two of them, though perhaps the other board readers here have some more creative solutions then I can come up with.)