Whether they disappear like an old friend you have lost contact with, or they leave a corpse saying that they have definitely gone to another dimension, people are going to shift to a dimension where they are close to the same level.
Many people use "3D" and "5D". Some will use "those who have gone to the world where we have alien contact, and those who have not had alien contact yet". But, it is really about vibration (The average frequency you are able to maintain). And your current vibration determines whether you will enter this future sooner or later.
And we are about to enter a time when the people leading the way are going to start raising their vibration a lot. And, when the vibration level difference gets too large, you can no longer see them.
Vibrational range
When thinking about vibrational range, it is a good analogy to comparing it to old, antenna based televisions. Channel 2 and channel 4 lived in the same space at the same time, but when you turned the knob to 2 you got one show, and when you turned the knob to 4, you got another show.
The show on 2 never interacted with the show on 4. However, if the signal was bad, or your TV's potentiometers were set poorly you could get bleed through from channel 3 into channel 2. Sometimes you could see both shows as ghostly images, or you might see a few seconds of 2, and then a few seconds of 3, flipping back and forth.
There is a lot happening on Earth right now. A lot more than most people can imagine. And it is all happening in the same space at the same time.
As a side note, if you practice raising your vibration, there are times when your friends can't see you. Many such people come back into the office, after a 15 minute break meditating, and their office workers don't see them. They have to call out, or similar before their friends notice them. And, there have been reports of people using this to hide. Actually turning invisible to their pursuers.
Vibrational range and aliens
The upper range of our normal spiritual practitioners, and the lower range of the aliens who are coming to visit are getting close enough for these two to work with each other.
The aliens who will visit first are those who are vibrationally closest match to us. And this will still require the visiting aliens to lower their vibration so that they do not burn out those who they are visiting.
AND!! there are aliens who are coming to visit who are an even higher vibrational level. So, yes, this group will have to wait even longer before they get to come say high to us.
People who's vibration is too low will not see the aliens at all.
Where are you?
This is a difficult question, because we really do not know how to calibrate our instruments, or exactly what to measure. However, there is a lot people working on taking vibrational images of people. Images that kindof show the aura. Or devices that can tell if a body is ill, and where.
The real way to tell is if you can see the aliens or not. It is also highly correlated that if you are like researching aliens, or going out to the desert to look for UFOs, that you are on your way to seeing aliens. People are of low vibration are usually not interested in aliens at all. They simply, straightforwardly, aren't interested at all. It doesn't even cross their mind to think if aliens are real.
It could be said that the lowest aliens are vibrating twice as high as high level meditaters/spiritualists, right now. And this is why many people are going to be disappearing. We are about to kick it into high gear about raising our vibrations. So, soon, probably even happening now, there will be people will disappear. Of course, those near the top will do what they can to pull up the next layer of people. And so on down the line. People will not be left behind, however, it will take some people a lot longer to get there.
So, if you want to be in the first groups to have actual visits with aliens, then work on yourself. Dive deep into shadow work. Meditate regularly. Exercise regularly. Every bit helps, and you also cannot leave a part of yourself behind.
There are no shortcuts, and no one else can do this for you. Money won't help, you can't buy your way in.
And, although being around highly spiritual people can help you to raise your vibration, this is no surety to getting to the level you need to be. There are many stories where people, have been out in the desert, in the car, or something, and one sees a flying saucer with little aliens getting in and out of it. And the person sitting right next to them sees a school bus with children getting in and out of it.
This is not a race where it is important to get across the finish line first. In fact, there is no finish line, it is just people moving to the next lesson. And you average vibration might not be enough to see anything, but someone with a similar level may have glimpses because they spike higher.
Continue your practices. As you raise your vibration, as you heal yourself, you are getting better; your whole life gets better, your body gets better. Old diseases fall away. It is very worth every step your make.