How can you determine which employees should be included in a firm's incentive plan?

I'm assuming you're asking this as some kind of management class question. Well, the management class would tell you that there should be some profit sharing involved for all the employees to keep them being good workers and to boost morale yadda yadda yadda. But really, in practice, only those with managerial positions need to have it because they would be the only ones to have any incentive to operate opposite to the best interests of the company. Lower level workers won't be more or less motivated either way because their share of the profit will be too small to influence their decisions. And the size of the company matters too of course.

I think if they have been there for a certain amount of time they should be included. Different companies put up different amounts of time.