Both adduser and useradd should be able to add new users; if you use adduser, then make sure your /etc/adduser.conf is ok.
You can get some
info about the logged-in users via w
commands, but not much. I'm not aware of programs that provide you with telefone numbers or such special information that may have been added, sorry.
Use "useradd" to add users to new groups; groupadd and addgroup are tools for adding groups to the system. Read
to get more knowledge on how these commands work and how they specifically differ.