You could follow a natural progression of our current technology and culture:
Present-day: People use smart phones to connect to to the internet, communicating and sharing information globally. Automatic translation software helps break down language barriers.
Near-future: Internet connectivity is now available as an implant, but the information is still presented in ways that we're used to experiencing it: through text or spoken communication.
Future: The internet connectivity implants are now almost entirely ubiquitous (safe when used as directed), connecting people from countless different cultures and languages.
At this point, automatic translation software no longer satisfies a society addicted to instantaneous information...it's too slow and imperfect. The solution is to remove language from the equation.
Far Future: The implants are now capable of transmitting abstract thoughts directly, without any intermediate steps of partitioning them into language first. Everyone now fully understands everyone else, always.
The Next Step: After granting perfect, language-free telepathy, the next step is allowing for shared experiences. Rather than just sharing a thought about the smell of a flower, someone can share the actual experience with whomever they want. With the whole world.
The Unintended Consequence: Possibly the greatest advantage of any intelligent species is adaptability; the ability to be shaped by our experiences and environments, rather than behaving like simple machines (I smell food to the north-- therefore, I move north). But now, all thoughts and experiences are shared from birth (forcing a child to live "cut off" from Society would be the most horrible form of abuse)...everyone becomes more and more similar over time.
Consensus: After many generations, all people would think as one-- and they would be bored. There would be nothing new or original, and uniqueness would become the most prized resource. The only way to mine that resource is to add individuals from other cultures who do not already have implants. At this point, the only such cultures would be extraterrestrial.
Assimilation: The first attempts at assimilation would likely be peaceful attempts to convert outsiders to a superior way of life (no war or unhappiness, and access to all the information you could dream of), but the Collective would instantly overpower any new "recruits" and essentially devour their distinctiveness.
Resistance is Futile: The peaceful method of assimilation would be too slow to satisfy the demands of the Collective. The experiences and memories of a few dozen individual aliens would instantly be fully analyzed by a few billion connected brains. The only way to truly add uniqueness would be to forcefully assimilate billions of individuals.
This leads to a feedback effect wherein the larger the Collective becomes, the quicker it "devours" the distinctiveness of any new members, and the less effect that distinctiveness could have on the whole of the Collective. Thus, even if they devour an entire planet of people who treasure individuality above all else, it would not be enough to sway the Collective from their chosen course.