I suggest you take a look at the ISite interface in the SDK, or more interestingly, where it is used. You can use SiteHandler to get the ISite object corresponding to a specific site that you have configured in the System Settings - Websites section of the community administration. Then use this object when getting and saving objects like clubs and forums (most *Handler get/save methods have an overload taking an ISite object) to utilize the partitioning.

Thanks.

 But can you tell me what are the benefits for Community Partitioning and how useful it will be? I try to search for this on net and I couldnt find out much on it.

Can you help me on this? 

I haven't used it myself, so perhaps someone with experience has more input. I can imagine it being used to display/hide content depending on a users language preferences, or to create multiple communities (for example for different departments within a company) with a common user database as well as some global content.

Thanks Magnus for the reply.

If that is the case, we can also achieve the same thing with Clubs in relate plus based on your idea.

Is it so? 

Relate+ is EPiServer Community, so you can use site partitioning. But beware that the number of sites available is a licence thing. At least the old pre-Relate+ entry licenses included only one site apart from the Global site.