On Ubuntu, there is a useful package called adduser.
But on Arch Linux, I searched it by pacman -Ss adduser, and got nothing.
I'm not sure is it in extra repository or user repository, or actually it's not in the arch linux ecosystem?
Is there another way to search it on Arch Linux except installing it directly from downloaded source code?
There is no adduser command in Arch Linux.
As the wiki makes clear, Arch uses useradd. And there is no need to install anything, as it is packaged as part of the [core] repository:
pacman -Fs useradd
core/shadow 4.5-4
In general, it is a good idea to abandon your reliance on how Ubuntu does things when you move to Arch. Arch provides all of the tools that you require to competently administer your system, and a package manager that suports you identifying which packages provide these tools.
And a wiki that meticulously documents them.
See this question for the differences between the two: What does adduser do that useradd doesn't?
To install the adduser command, use:
pacman -S yaourt
yaourt -S adduser
Note that yaourt is deprecated. You may now add users using adduser.
A reasonably equivalent command is:
useradd -m -G groupname -s bash username
When you have some spare time, study the documentation.
