Considering Joining as either:

A. something akin to the borg from startrek and a bit of the Geth from mass effect, however unlike the borg they retain individuality (unless the species that joined them never developed individuality, and also people can elect to join hiveminds within the civilization) most of the population exists in virtual reality worlds completely incomprehensible to human minds. The "civilization" began with Space Workers of some ancient empire responsible for maintaining FTL communication and the galactic internet, over time other less advanced cultures wanted access to the internet and over time became integrated with it. Recently a group of human cyborgs joined the network and regularily interact with other cultures. While the rest of this society has no government or agency that humans can understand, the Human portion of the network still acts fairly human (for now, it will inevitably retreat with the rest of the civilization to become either nomads thinking on galactic timescales or solipsists never leaving the comfort of virtual reality)

B. Or a militant race of insectoids protecting themselves from a far superior technological civilization using relatively primitive tech, this race conquers other lesser species in hopes of feeding its war machine. While they enslave other species most species they enslave end up living far happier lives with increased technology.

I could make both empires, the first one being more Aloof tricksters occasionally visiting planets and stuff while the second one could be more involved with galactic affairs.