I played with a GM who would only give XP for people who showed up, and after a while it was a bad time as it became more and more difficult to create encounters that were challenging-yet-survivable for everyone. In a cooperative game, making some characters significantly less effective also punishes the rest of the group. So I really wouldn't advise that, even if it's suggested in the rules.
Your course of action here really depends on what you want to achieve. Ideally, the game is fun enough that everyone wants to show up every time, so that by itself is enough incentive for people to come when they can actually come. If you're dealing with some flaky folks, some form of "punishment" may be appropriate, but especially since you said the absences are due to work, I really wouldn't do that in this case. So I would give everyone the same share of XP (or, as some others have suggested, just level up the group at appropriate narrative/time intervals.)
I can see how, as Cyberspark says, that could feel like they're getting all the rewards with none of the risk, and that could feel unfair. But that needs to be talked out in your team. Again, I would argue that the real reward of showing up is that you get to play and participate in the story, and when you are all together again, you don't want to deal with some members of the party not being able to effectively participate. But if that's not enough, you can:
- Give out minor rewards - not XP, but favors, relationships with groups or individuals, nifty trinkets, etc. - preferentially to the folks who are there, as long as it makes sense within the story. (Significant rewards, like "The wizard gives each of you a magic item in return for completing the quest!", should still go to everyone.)
- Take the opportunity to do some deep character development. Give the present players a few flashback/monologue/romance/decision scenes where their character is really the focus, when there are fewer people around to get bored if they're not involved.
In this way, the characters of the players who show up will eventually be more fleshed out and have slightly more influence on the plot of the game, while no one is actually hampered by significant drawbacks the way they would be if some characters were lower level. And it's also not a punishment that you're explicitly doling out to people who really couldn't help the situation; just the natural progression of things.
By the way, you should also discuss, among other things, how deadly the adventures will be and how absences will be handled - in a party of four, if one or two characters are conveniently elsewhere for an adventure (one of the standard ways of handling this), the rest could find themselves unable to accomplish the task at hand. But if you include everyone and play the "missing" characters yourselves, you'll have to either A) risk death or other serious setbacks to a character while their player is away, which can be quite frustrating; or B) go through some amount of inconvenience as a GM to make sure there's no real risk to those characters. How much of a problem this is depends on how risky your game is to begin with, but it's definitely worth establishing in advance so no one is caught off guard.