In general, the game can be played solo without missing out on content. Multiplayer aspects include player associations, PvP combat, trade, legal situations and ventures. So there are many aspects to the game where player to player interaction is passive, meaning features of the game cannot exist without having multiple players in the game. But it doesn't mean you have to actually interact with the other players.
Call them what you like, but the game supports multiple players banding together into an organization. Rules are simple, you need at least three players to form a guild, the guild must select a faction to join and the stellar system in which the guild is established becomes the home system for the guild. New members must travel to the home system to join.
Guilds can be open or private. Open guilds allow anyone to join and private require special invitations from high ranking members.
Guilds also offer a simplified ranking system with significant flexibility. Guild leadership is given to the player that initiates the guild. The leader can proxy ownership to any other guild member which allows them to act as the leader and can do anything the leader can do except disband the guild. Guild leadership is then able to establish the ranking system they will use. Basically it is a number tagged with a word. Rank 0 is the guild leadership, guild leaders can then decide what the label for rank 0 should be. On creation of a guild, a default level 10 is included which carries the label "member". Leaders could setup ranks in any manner they wish and can also assign control over guild functions to each rank. For example, a guild may form a rank of 4 and label it "Captain" and then assign that anyone in this rank has the ability to allow players into the guild.
Player versus Player combat will be implemented in the future. The concept behind it is fairly simple. Players must own their own ship for combat and they must also declare themselves a member of a faction. Factions are essentially NPC Guilds. Once a player is aligned with a faction, they can then turn on the faction signal in their ship which will then open them up to PvP combat with players in the vicinity that are aligned with other factions. Small warning, the person who is writing this game sees nothing wrong with ganking and is actually designing methods for stealth to make ganking even more fun.
PvP combat does not result in death. Combat ends when a player's ship is incapacitated to the point where it requires significant repair. The player that inflicted the most damage gets looting rights and may strip components off the disabled ship as well as take items from the losing player's inventory onboard the ship. The loot is randomized and is in proportion to the amount of damage inflicted. Note that damaged ship components stay damaged, e.g. the player that loots a broken shield generator will have to get it repaired if they want to use it. Players interested in PvP combat should definitely consider insurance.
One of the more interesting features the game offers is the ability to control a single space ship from multiple accounts. There are a few ways you can use this feature. In developing the game, it allowed me to run two instances of the game so that I could fully utilize two monitors. On the main monitor I have the main navigation computer up and running, on the second monitor I'm making trades in the stock market. If I had a third monitor I would likely have the screens for my mining operations.
Another way to share interfaces is to use a modem. Ship owners can purchase a modem for their ship which then shows up in the main COMM panel. It is possible to have up to five other players dial into your ship's model. All they need is a special code that you supply to them. Then you can configure what parts of the ship they are permitted to control.
To put this type of ship sharing into perspective, combat is complex. Besides firing weapons at other ships, you must control your ship's navigation, manage the power systems, manage the life support systems, respond to emergencies and also manage your defenses. Even with some good scripts running in the terminal and a well buffed ship/crew, you may find you still need some help. Fire up the modem and let a friend handle some of your systems.