SolvedCSE2305 | General provisions: Can a UK established CSG have members in other Member States (OMS) or a country not within the EU and can a UK business or organisation belong to a CSG established in OMS or a country not within the EU?
From HM Revenue & Customs · VAT Cost Sharing Exemption Manual
No. The exemption only applies to services which a CSG supplies to its members established within the same member state (see CSE4120). With effect from 1 June 2018, the exemption is not permitted where members of a UK established CSG are established in other EU Member States.