Creating multiple node references I have the following content types - Efforts, Objectives, Groups and the user.

Efforts and Objectives can be shared by both users and groups. A user/group can have multiple objectives and efforts and can also share the same objectives/efforts.

Is it possible by using node references ? or Is there any other way where I can implement this functionality in drupal ?
sure you have to use node reference
I think you need a node reference in efforts and objectives to select users or group so you need a node reference to a view (you can reference to a view it should not be node) 

the same user/Group make a view reference to objects and efforts

If you have any problem make a comment