Marbits are united in a non-capital side and apparently are Natural Allies to Jetstone.Erf-b1-p097 They are one of several Natural Allied sides taking part in Royal Crown Coalition and sent 1875 troops to the RCC, second in number only to Jetstone.Erf-b1-p086
Marbits, as a non-capital side, must earn money to pop new units and pay upkeep. They most likely accomplish this via mercenary work/alliance-based salary, and mining, to which they probably receive a significant bonus. They may be able to be used as engineers, digging tunnels and the like, in addition to having tunnel-fighting skill. These advantages likely balance out their small stature and likely low overall stats.
In the Battle for Gobwin Knob, Marbits were mostly light units mainly seen in the tunnel fights, especially as scouts for Webinar's forces. They were routed by Sizemore and Gobwin forces. It is assumed that none survived the Volcano uncroaking.
Like several of the other factions in the RCC, they had a commander at Ansom's table, and it may be assumed that it was a warlord. Erf-b1-p052
Marbits have not used coherent speech insofar.
Real world Reference
"Marbits" from Parson's world are an abbreviation for "marshmallow bits" that are a component of breakfast cereal Lucky Charms.Erf-b1-p015 When Wanda is summoning Parson, Stanley asks for someone who (among other things) snacks on gwiffons and eat Marbits for breakfast.Erf-b1-p017 Apparently Stanley gets what he asked for.
Appearance aside, marbits seem to be an abstract of the classic dwarf; able miners, living underground, and at odds with the goblins they encounter there.