In Only In Death, we're given this description of the Blood Pact's rank structure:
The encampment commander held the noble rank of damogaur. He had absolute control over eight sirdar brigades, and answered only to the etogaur of his consanguinity, and the Gaur who ruled above them all.
So, from the top, we have the following:
Urlock Gaur, effectively Lord General. Note that "Gaur" is sometimes treated as a title, as in the above quote, and sometimes as a name. Note also that Gaur is currently the Archon of the Sanguinary Worlds, making him the equivalent of the Warmaster, but his Blood Pact and its titles predate that arrangement and were actually a major reason why he became Archon.
Below Gaur are the etogaurs, who command a consanguinity. This appears to be a very senior position, and is explicitly compared to an Imperial Guard general.
Below an etogaur are damogaurs, who command a philia. The general description of the position and its status as someone who could be found near, but generally not on the front lines would suggest to me something like a major or colonel rank.
Below the damogaur are sirdars, which are a bit anomalous. When the encampment that the damogaur above commands is first described, the narration notes its high-gain communications gear and suggests that indicates a sirdar rank or above. So they have some command authority, and their brigades can be fairly sizable (the encampment is estimated at about a hundred). From this, I would guess about a captain-equivalent rank.
On the other hand, on Gereon, there is a sirdar seemingly in command of a single squad. From Traitor General:
The patrol commander, who bore the distinguished rank of sirdar, ... check[ed] his team's disposition.
The text suggests about 15 soldiers, all of whom fit in one truck. It's possible a captain would command such a small force, albeit irregular. However, it's mentioned that Uexkull, a senior Chaos Space Marine, is coming to oversee the operation in person, so perhaps the sirdar thought that a brief stint in the field would be a good career move.
When Uexkull arrives he deals with the three senior officers of the "region" (perhaps province-sized, judging by the time it takes the Ghosts to reach the neighboring region), who are a chief sirdar and at least two vice sirdars. (Apparently our by-now-deceased friend the regular sirdar doesn't make the "senior staff" cut.) Depending on the size and dispersal of the garrison forces, captain wouldn't be an inappropriate rank for this sort of command, nor would major.
If there's any defined rank below sirdar, it's not named by any of the books I checked.
Bear in mind that these are first and foremost Blood Pact ranks. (According to Blood Pact, the Sons of Sek use essentially the same ranks, being organized by Pact defectors.) There is no common rank structure for all Archenemy troops; they're called "Chaos" for a reason, after all. Damogaur and etogaur in particular are identified specifically with the Pact in Traitor General; sirdar appears to have wider usage, as evidenced by its use among non-Pact or Sons occupation troops in that book.