@Enodoc and @RaddlemanNumber7 are right, OP. I'll just write a bit more to support their words. The semi-nomadic Orcish tribes are a society very similar in it's type to the Dunmeri Ashlanders' one, though with only one exception - they are at their final stage of the primitive, tribal, type of society and are very close to feudalism. The communal relations between relatives are still very strong but the social inequality and the division of labor begin to differentiate common tribesmen from their chieftains and kings. The Orcish type of society and their city of Orsinium are similar to the society type of the semi-nomadic Turkish Oghuz tribes especially its Seljuq branch of the XIII century and the Sultanate of Rum IRL respectively. The Orcish relations with the Breton feudal kingdoms and the Empire are also very similar to the Seljuq Turks relations with the Crusader Kingdoms and the Eastern Roman Empire IRL.

Ranks or military titles, if you meant those ones, are common to regular armies and, as you have mentioned it, the Orcs do not have one yet, thus there are no purely Orcish ranks except that "Chief" title. Even the chief's assistants are called simply.. assistants like that Chief Gloorot before he became the chief. But yes, @Enodoc and @RaddlemanNumber7 are right - some distinct military units use those ones common to us. In order to create a regular military organization the Orcish society has to:
1. Develop it's social division of labor further on;
2. Create the two main classes of feudals and peasants where the military estate would be a feudal class's estate.
3. Unite the clans under the rule of feudals with a high king;
4. Conquer territory to establish an Orcish state.

All these four terms are equal in their value and must be achieved in order to build an army you speak of. The constant building and destruction of Orsinium is one of the things that prevents it.
Guilds are the product of late feudalism: legalized in 2E 321 by the Guild Act they are the first capitalist companies established in the Second Empire for financial gain instead of subsistence production, they are too far ahead of the Orcish society. If you wish to create an Orcish guild, you could use something other than ranks, something corresponding to the very beginning of the Orcish social devision of labor: a chief as a head of the guild with his assistants, hunters, gatherers, merchants, warriors and smiths as other equal or almost equal "ranks". You could also make some "wives ranks" such as "forge-wife", "hunt-wife" and etc., but I doubt too many girls play the Orc characters. Or you could also use those traditional Alliance military ranks @RaddlemanNumber7 told you about, if your guild is something like a private military company abroad fighting for an Alliance. But there are there 26 ranks.. Anyway, those ranks depend on the type of guild you have created. If you could provide us more information on your guild, we could offer you some other more precise ideas.