Document Acknowledgement and Attestation with Microsoft’s PowerPlatform - Step by Step

This is old to the point that I don't know if I bother posting, but I'll post in case it helps anyone in the future. 

I had trouble with the array, as well, so I did that part a little differently than Michael.

Assuming you're requiring attestation from a group, you can run a compose function with 

replace(variables('RequiredClaims'), 'c:0t.c|tenant|', '') 

(In my case, the I've put the group that needs to attest into a variable I've named "RequiredClaims.")

This will get you only the group ID of the group assigned. Then, you can run "List group members" against the output of that compose, then your "Apply to each" loop will run against the body/value of the "List group members."

To address the issue of MrJohnson1905 about the later "Start and wait for an approval" hanging on for each member, go to the Settings section of the Apply to each loop and turn on concurrency control. Even though this seems to imply it runs infinitely concurrently unless enabled, in testing I found that it does hang up for each user unless this limit is set to "On." The maximum degree of parallelism it allows is 50.

Working from there, I inserted a "Get items" against the attestation list as the next step in the "Apply to each" loop with a Filter Query of "Title eq '(whatever you're putting for the title of the attestation list item)'"

Follow this with a Condition of length(body('Get_items')?['value']) is equal to 0. This checks to see if the list item already exists; if this expression is true, the item does not exist, and we can move on. This takes care of the issue of creating multiple list items, as well as the issue of new starters in the middle of an attestation season.

THEN you can create your item and Start and wait for an approval.

Despite the last comment being more than four years old, I'll tag those that had similar questions to mine as I've worked to try to implement this:  Spetschristopher AshleyMallory. Apologies to the remaining people; this site will not allow me to tag PamKaiser-Rabobank, PachJugpal, or Ciprian_Adam, for some reason.