In the example above, the company is making the flu shots available to their employees. So you can refer to them as "the flu shots being offered".
It might help to understand this if you imagine that some words have been deleted:
the flu shots (that are) being offered (by the company) this week
As another example, imagine that you're a member at a gym. You can say:
I should probably look into signing up for one of the classes being offered at my gym.