Xcode should be the default application for .xcodeproj files, so this should work:
$ open a.xcodeproj
If that opens a different application, you can force it to use xcode:
$ open -a Xcode a.xcodeproj
If you want the command xcode to work, you can just alias it:
$ alias xcode="open -a Xcode"
then you can just xcode a.xcodeproj (and add this to ~/.bash_profile)