not sure where you're from, but having just gone through the same situation, here's what i learned.
the immigration authority (IAS) AS WELL AS ministry of manpower (MOM) issue the same types of visas. the difference is that application through IAS is based on your g/f's own merit, whereas application through MOM is attached to your EP.
your g/f will need to apply for a 'long-term social visit pass' (LTSV) through MOM. the LTSV is catered for common law spouses. in order to apply for the LTSV, you need:
1) a local sponsor that can be a company (yours preferably) or an individual (has to be PR or citizen)
2) a letter from your embassy/high commission to recognise your relationship as a legal one (i'm told that the british high comm no longer issue the 'spouse letter' as they have an arrangement with MOM, but someone forgot to tell MOM about this! so alternative is to get your company to issue this letter)
3) a statutory declaration from you to acknowledge your relationship with your g/f, preferably stating length of relationship and living together
it would be faster for your company to submit the application online, as there is some 'online e-govt thing' for registered companies. they say that online application takes about 2 to 3 weeks, whereas counter application takes 3 to 4 weeks.
you better start the application now! good luck...
