You need to get in touch with your COST National Coordinator (CNC), who can nominate you either as a Management Committee (MC) Member or, if your country already has two representatives, an MC Substitute. Find out more at the COST website. If your country has more than 2 MC substitutes, it is preferable for you to join as a Working Group member. For European WG membership, please sign up at our registration page. For international involvement, please email email@example.com for details on the application.
WG members are entitled to attend all meetings except the MC meetings, where MC members get to have a say on matters regarding the running of the Action. Other than that there is no difference - eligibility for reimbursement for meetings and for STSMs, conference grants and dissemination grants are judged irrespective of position within the Action.
COST regulations will not allow reimbursement for Action members from international institutions (outside of the COST European country list). This means that they cannot be funded to attend meetings or carry out STSMs, but European COST researchers can gain funding to perform STSMs in international member institutions.
ESR stands for 'Early Stage Researcher'. The COST definition of an ESR is a researcher who has completed less than 8 years of research after completing their PhD.