The REST API in ALM 11.0 (the version you have) is focused on objects stored in Quality Center / ALM, such as requirements, defects and tests and associated history (audits). You will have to use the ALM Site Admin API to add user to a site, to a project, to a group, etc... Site Admin API reference for ALM 11: http://support.openview.hp.com/selfsolve/document/KM997954
Note: the user has to exist in the site admin schema so that it can be added to a project.
Starting with ALM 11.5, there is the possibility to add (PUT) a user to a project. Reference - ALM REST API 11.5: http://support.openview.hp.com/selfsolve/document/KM1413621 In general though, administrative tasks are best covered by the site admin API than the REST API.