Even through you may not be married, if you have been with your partner for over two years you can make an application for them to join you in the UK.
The requirements for successful applications are similar to those for spouse visas however, the main difference is that you must provide evidence that you have been in a relationship for at least two years.
If you are applying from inside the UK, the requirement further states that you must show evidence of co-habitation (living together) for the past two years.
Apart from this requirement, you must ensure that you meet all the other requirements in the Immigration Rules including the financial requirement.