There is a joint research unit in France where staff can be in the same unit but have different employers. If only one employer is a beneficiary in the consortium but some staff from the other employer would like to be seconded, does the other employer have to be added as an associated partner linked to the beneficiary?

Yes, the type of link and the activities of the Associated partners linked to a beneficiary should be indicated in Part B, and they will be assessed as part of the evaluation.