Q: Does each legal entity need to be a partner in the proposal or can the EU legal entity be the only partner from this company and use the resources of the others as long as the costs are recorded in the accounts of one of the companies?

A: In principle all other legal entities carrying out part of the work should be signatories to the Grant Agreement. The only possible exceptions are in the case of subcontracting (with clear rules as specified in the GA), and entities covered by special clause 10 which refers mostly to affiliate companies and Joint Research Units, EEIG and other Groupings. This is explained in The Guide to Financial Issues (p. 33-34).