SolvedCSE2300 | General provisions: Can a UK established CSG have members in other Member States (OMS) and can a UK business or organisation belong to a CSG established in OMS?
From HM Revenue & Customs · VAT Cost Sharing Exemption Manual
Members of a UK established CSG can themselves be established in other EU Member States. Similarly a UK established business or organisation can be a member of a CSG established in another EU Member State.