"Users" of Limesurvey are those that can create surveys (survey admins).
Why would you want to group them? Because you have different groups of people in your company/organization and not everyone is supposed to do everything / see everything.
E.g. there are different rolls in the company. Some people are not allowed to change templates/themes, but they might be allowed to see results.
Or employees of location A are only allowed to see surveys for location A, not B.
Only 2... no use for any more at this point...
Well, if you only have 2 users in your installation, and according to your screenshot, they are both in the user group 4 already. So of course the dropdown is empty. You have no additional users to add, they are all in the group already.
Also, as i see it, i CANNOT group PARTICIPANTS (what i would "normally" call a "user") (who came up with these terms??)... there isn't even a field to put a key of some sort, so how can you have nice "sets" of participants??
I think "user" is fine here. Of course you would consider respondents/participants somehow a user of Limesurvey, but the survey admins are more of a user of Limesurvey to me than a respondent. But anyway, what causes confusion is the two different terms of "survey admin" and "user" used for the same thing.
In any way, I personally wouldn't call respondents/participants "users".
But, don't forget that Limesurvey is open source and has been growing a lot in the last couple of years. A lot of different people have contributed to this and especially in the community driven translations you will often see inconsistencies in translations. However, as it is open source, you are welcome to help. The manual is a wiki which you can access with your credentials of the forum and you also can help to translate limesurvey under translate.limesurvey.org.
If you feel that something is labelled wrong, you can make suggestions and bug reports in the bug tracker. However, in your case I (and probably many others) would downvote to call participants/respondents "users", for above mentioned reasons. What I would support is have a consistent naming convention for admins / users. Which of both terms, I don't really care. They both fit. It just should be consistent.
Maybe we should suggest that they add "participant groups" (or do they have this hidden somewhere??)... that would seem a lot more useful than groups for 1-3 admins (oops, sorry... USERS LoL)
There is no real "user management" / "participant management" in Limesurvey, like you expect it (with login and password). For participants you can use the CPDB (I don't know much about this because I hardly had a look at it) or you use token tables (these are on a survey level). But this is more like a list of respondents, no login for them.
What you are actually looking at is often not part of survey tools anyway: You are looking more for an online access panel solution, where you organize respondents for surveys.