I too have seen some conflicting articles on this. In general, I use it as follows:
If I'm using people as the plural of person, then I treat it as plural.
These people are going to be the death of me.
If people is referring to a community, cultural group, etc. then I treat it as singular.
This indigenous people was subjugated by invaders from afar.
I'll also note that used in this last sense, peoples (meaning multiple cultural groups, or what have yoU) is also valid and would then be plural.
These indigenous peoples were all prospering at the turn of the century.
As Sam said in his comment, the crux is what precedes the "this/these include(s)", not what follows. As a matter of fact, "This includes administrators and normal users" can be just fine, depending on the context.
That said, these can still be a little tricky. For example:
Several different users can change their own passwords; these include administrators, and normal users.
The "these include" refers to "several different users." But:
No users are allowed to change their own passwords; this includes administrators, and normal users.
In this case, the "this includes" is singular, because it refers to the single principle that no user is allowed to change a password.
Corruption was uncovered at many levels across the organization; these include both administrators, and normal users.
Here, "these include" refers back to the many levels where corruption was found.
However, instead of trying to figure out the correct usage, a better option would be to use the more all-encompassing including. In fact, this change seems to improve the sentences:
Several different users can change their own passwords, including administrators and normal users.
No users are allowed to change their own passwords; including both administrators and normal users.
Corruption was uncovered at many levels across the organization, including administrators and normal users.
Get rid of this this, and this problem goes away.
A footnote (to underscore how tricky the this/these choice can be): I could have said, "In fact, these changes seem to improve the sentences..."
Which is correct? If I am referring to the single hint of replacing the phrase "this includes" with the word including, then "this change seems to improve" is appropriate. But, if I'm referring to the three individual improvements found in each example, then "these changes" would be better.
Best Answer
In the context of your question, "all this" means many things taken as a single whole. "All these" means many things as part of the whole; not everything.
For example I might be a boss pointing to a room full of boxes and say, "all of this needs to be moved into the next room." I can use the word 'this" because I mean everything. It is taken as a whole.
If I meant only some of the boxes, or if a mistake could be made if I'm not specific, I would point to specific boxes I wanted moved and say, "all of these need to be moved into the next room."
Another example I can think of is something I can picture from the movies: a rich guy looking out at all the land he owns with his son beside him. He waves his arm and says, "one day all this will belong to you." He means everything. The whole. Even if all his land is made up of individual items he would not say, "all these will be yours." It might be correct, but it also implies he is only talking about certain parts of the things in front of him. The son would ask, "which of these will be mine?"