M-dash

The em-dash is typically used to replace commas and provide a greater emphasis to the pause than the comma provides. The em-dash may also replace colons and semicolons. Let's look at your first example to see how it works.

Could you please provide me the full list of potential participants--I would like to be able to track absences. (Note: nearly all typographers agree that no space should occur before and after the em-dash.)

The sentence is best expressed (and punctuated) as two separate sentences. Could you please provide me the full list of potential participants? I would like to be able to track absences.

The two sentences can be combined into a single compound sentence. The most common (and preferred) method is to use a comma + conjunction. Could you please provide me the full list of potential participants, because I would like to be able to track absences. I dislike this because the question mark is lost, and the meaning become more vague. This may be a bit better: I would like to track absences, so could you please provide the full list of participants?

Although the comma + conjunction formulation is the most common, you can also use a semicolon if the two sentences are tightly linked. Could you please provide me the full list of potential participants; I would like to be able to track absences. This still has the problem of the lost question mark.

To provide greater emphasis, you can replace the semicolon with the em-dash. But again, you still have the question mark problem.

Your second example presents a different issue. Technically, the em-dash can be used to replace the comma that ends the phrase beginning with "Regarding ...."  What you say in this sentence, however, does not merit the increased emphasis of the em-dash.

The em-dash is a useful tool. It should, however, be used sparingly. If overused, it loses its power to emphasize. I hope this helps.