Original: Proceedings of the National Academy of Sciences 104: 9736-9740 In this case, an individual's inclusive fitness varies depending on how much it invests in within-group competition (e.g. hoarding a private food cache) versus between-group competition (e.g. contributing to common foraging); and on its relatedness to the other group members.
Simplified: In their model,  an individual's inclusive fitness varies depending on how much it invests in within-group competition (e.g. hoarding a private food cache) versus between-group competition (e.g. contributing to common foraging); and on its relatedness to the other group members.