I'd tend towards a group advancement system simply because it's easy.
Say that at each level-up the players get d5+2 points (3-7 points, randomly picked for every seperate character).
If you do at one point decide to give players a bonus, grant a pair of extra points or roll twice and pick the best result.
Since the group level-up is more or less periodical there's no need to track progress or achievements to determine just when an individual can 'advance'. All you have to do is review whether or not you have a good feeling about the way a player is taking his character.
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