Independent:

Small groups of 1-10 people who build up a base alone and then others build a base nearby and they ally up to strengthen their position. They will be very simplistic and will have few defences against Rebels. They may go to trade with other independent bases or Imperial "Cities" however will rarely actively hunt rebels. They may also move around so they are not in danger of being found by rebels

Rebels:

Groups can be any sized. They will generally have hidden bases most likely underground. They will craft many destructive things so they can raid Independents. However after some time ,like the Independents, they may grow to imperial size and then they will be a very dangerous force. Imperials will be trying to destroy them. This means that to the Rebels, Imperials will be the Greifers. Also rebels might not destroy Independent camps. They might leave them alive and offer them protection to make them gain an Imperials trust so they can sneak things inside ,such as TNT, when pretending to trade.

Imperials:

Groups can be any sized. They will generally have large bases to act as a beacon to other players. They will build strength much faster than rebels because of this fact. However because of their large base they will rarely move. This mean they will have to rely on trade from Independents to continue providing them with resources. They will actively hunt Rebels partly because they are an annoyances. But also because their dug out bases can act as useful mines. Without the hassle of having to dig them them selves. In times of need of if they just think its easier imperials will just attack Independents just because it's easier than having to give them something for what they need.

 

Various Jobs

Traders:

They will be useful to everyone as they will ferry reseources around. While they won't earn anything from this they will eventually be able to afford extremely expensive weapons. Also if they are living in a large settlement this might be their way of paying their rent. May also have armed support.

Bandits:

Rebels who's job it is to intercept Traders. This can be difficult as traders with anything worth taking will probably have they own group of Guards. A group of bandits may also pretend to be Guards so that people hire them short term. Then while they are in the middle of nowhere they will mug them. Or they will wait till they head to a independent trade hub and raid the whole place.

Spy:

They will gain Intel on bases so they know where weak points are. They might just act like traditional Greifers too simply by breaking the walls downs from the inside before an invasion. They called also invade Independents settlements and try to convince them to move to a Rebel base or an Imperial city. 

Harvester:

Will leave the safety of Bases or cities to mine for resources without the premium of the Traders. If they find particularly rich areas they may set up a sister base or city. In which they will become the ruler presumably. Which also brings to light another point....Wars(see bottom(Lol))

  

Builder: 

Explains itself

Soldier/Guardsman: 

Explains itself

Wars:

 Imperial wars: 

They will happen simply because people are bored and find them fun or the same reason that Imperials will attack Independents. However it could also be so they are the most powerful Imperials on they sever. However this mean that all the Rebels will focus on them.

Rebels Wars: 

The same reason Rebels attack anyone else. For new recruits and more weapons to kill people with. Especially as Rebels bases have seasoned rebels. The wars may just be a show of power to decided which rebels king get to rule the joined forces. Rebels wars will be much lower key to the untrained eye as they will have many small battles under the ground where people break through and start a small skirmish at the point of entry. Above ground fights will be rare as rebels will probably patch up hole into their base if they see anyone coming. 

Independent Wars:

Small skirmish for Mayors of towns to gain more people or resources. Weapons will generally be sticks, hands or bows. Very little strategy will go into it.

That is why Survival Multi-player will kick-ass even with "Greifers"