This won't be a game where only the largest guilds can succeed, according to Moll, the Earthrise Community Manager. She explains that while territorial conquest will not be an endeavor best suited to small guilds, much less solo players, these smaller groups will be able to tap into a steady stream of resources and income. One way is to provide services to the playerbase and even the larger guilds.
Moll writes: "Playing as mercenaries, aiding strategical partners or providing guilds busy with preparation for the next siege a security convoy for their resource-loaded mecha transport for a cut from the load is definitely among the scenarios that can come to mind." Also given that siege warfare in Earthrise will require large numbers of players, there may be opportunities for these smaller guilds to offer their services as hired guns -- either as additional protection or as contracted strike forces.
If you're following along with the development of Earthrise or are interested in sandbox gameplay in an MMO, give Moll's "Guilds, Small and Large" a read and see what the game's community has to say about the roles of small guilds on Enterra.