I have questions:
1. If I create a user account name: user1 & passs=123456, Now, in the proftpd.conf file I put like this:
................
UserAlias user1 user1ftp #set alias user for user1
........................................

If I ftp into this server and use user1ftp & pass=123456, will it point to user1's home directory?

2. I want to setup /home/ftpstuff/ for 2 user(ie. admin & reguser). I want useradmin have full read/write to the directory /home/ftpstuff/download & /home/ftpstuff/upload and I also want user reguser have READ ONLY to /home/ftpstuff/download and have full rea/write to /home/ftpstuff/upload

My question are:
a. How can I create 2 accounts to access to same directory(in this example is /home/ftpstuff/)?
b. How to setup to do like what I said above?

Thank you

2 users can share the same home directory with no problems.

Have the admin the actual owner of the directory, with full access, then have the second user assigned to the group that has access, with whatever permissions you want assigned to the directory for the group. It basically all comes down to assigning ownership, group ownership and permissions of the directories.

man chown
man chgrp
man passwd
man chmod