By default if you install archlinux, there's only one OS to choose (Archlinux) on GRUB. To add another OS if you want to dual booting, the easy way is using OS-Prober. This will automaticly detect another OS.
OK just follow posts below:
1. Install OS prober
#pacman -S os-prober
2. Run OS-Prober
After you install it. Then type os-prober, i have PCLinuxOS and OpenSuse installed on /dev/sda1 & /dev/sda3. os-prober will automaticly detect it. If it's not detected, try to mount "not detected os" partition first. Then os-prober again.
$sudo os-prober
3. Then Update the grub system.
$sudo grub-mkconfig -o /boot/grub/grub.cfg
Now another os can be boot using Archlinux Grub2. On my machines, PCLinuxOS, and OpenSUSE run fine.
Enjoy.