Let's start by clearing Fenrir Greyback: Although he is an extremely valuable asset for Voldemort, the main problem is that he's a "Filthy Half-breed". Of course, this is not expressly directed at werewolves when said. However, we see throughout the series the discrimination they receive from the "normal" witch and wizard culture.
"But then Dumbledore became Headmaster, and he was sympathetic. He said that as long as we took certain precautions, there was no reason I shouldn't come to school...." (PA18)
I mean, it took Dumbledore alone to get a Werewolf into Hogwarts, which also means that Fenrir likely didn't go to school, which could be another part of his reason, but i digress.
I have a few theories on this, which can be separately responsible or may each have a part to play.
We know that Pettigrew was a wizard albeit a somewhat limited one. It is likely that Voldemort never witnessed his magical prowess or there lack of. Until the time of his resurrection, it is hard to believe that Voldemort had seen Pettigrew cast any spells at all. It is possible to believe that with Voldemorts lack of knowledge and his graciousness at Pettigrew's betrayal he gave him the Dark Mark.
To stop Pettigrew from being able to betray Voldemort, he (Voldemort) branded Pettigrew. There is likely one thing Pettigrew would fear more than Voldemort (at the time), and that would be Azkaban. This would help to ensure his loyalty through cowardice.
It seems, based on Sirius' confrontation of Pettigrew, that Voldemort either cast him aside or allowed him to return to his life in order to spy on the Order and his old friends. Now we see another possible reason which is to contact Voldemort for information.
Let's be honest. When you meet Pettigrew, it is incredibly obvious that he is an utter coward. It is more than likely that Voldemort gave Pettigrew the mark as a way to monitor him or to keep him safe. It is even likely that Pettigrew required the dark mark before he would snitch as a way of keeping himself safe.
On the whole, you can see that there are a lot of reasons for the the whole branding of Pettigrew. He is not pure-blood but then neither is Voldemort himself or of course Severus but this could still be an against for his branding. Other than the saturation of his death eaters I can see no other strong negatives for branding Peter. After all it serves a purpose.
If he received the mark after the first reign
Then I think it's fairly obvious that he merely used Pettigrew as a way to broadcast his return to the Death Eaters. We will have to presume that Voldemort is the only one who can give the Dark Mark, it is complicated magic. Although Barty Crouch Jr is a capable wizard and could be taught how to give the Dark Mark during his time with Voldemort around the GoF. So it is possible that he did indeed receive the mark after the first reign, this however makes the answer much shorter.
To summon the Death Eaters. He was a whatsapp group message.