The EC prefers to include study sites as partners. Other options are to subcontract the task (Attention: Only minor tasks can be subcontracted), to include them as In-kind contributions provided by Third Parties against payment or affiliated entitities and third parties with a legal link to a beneficiary. Please refer to slide 29 and 30 of the first presentation (webinar on 18.3.2015) for details. If you need to add study centers during the running project, it depends on how you have included them in your project. If you need additional partners, you have to file an amendment.