I have an XP/Redhat/Solaris triple boot in front of me. Sun modified grub to be able read their disk slices. You need to boot from sun's grub or chainloader to it. I am set up to boot from Solaris, but I have a floppy with grub on it. I can boot from the floppy and edit a menu item to read:
rootnoverify (hd0,2)
chainloader +1

Then I boot that entry and I see the grub menu from the solaris install. If I didn't know it was 2, I don't know any way to figure that out via grub commands. But Solaris only installs on a primary so it must be 0 or 1 or 2 or 3. Windows used up 0. I installed redhat on a secondary so 1 became an extended. 2 came next and that is what Solaris got.

Once you boot Solaris and the OS, not grub, is running, use installgrub to fix your mbr. Read the man page first. You will use -fm when you run it.