Dominance is the behaviour displayed by animals in a group in order to confirm the hierarchy in the group.
In guinea pigs, the most dominant pig will chase and mount the others. They may also rumble and sway at them as well as raise their nose higher than any other pig in the group. This might also be accompanied with teeth chattering and puffing their fur up. You tend to see that other members in the group move away or back down from the most dominant pig.
Whilst dominant behaviour is seen most in teenage pigs or pigs that have just met, it should settle within a few weeks. That being said a certain degree of dominat behaviour will always be present. It also depends on the pigs as well. If you end up with a grouping where more than one pig wants to be dominant, you may see more dominant behaviours as they try to challenge each other. Generally, as long as no blood is drawn and everyone in the group is healthy, there is no need to seperate the group.
You can help ease tensions between guinea pigs by making sure that there is enough of everything for each pig (food bowels / water bottles / hide-a-ways) and making sure that hide-a-ways have two entrancing so no one can get trapped or cornered in one.