"Staff members are researchers (Early-Stage Researcher and Experience Researcher), administrative, managerial and technical staff supporting the research and innovation activities of the project. They must have been actively engaged in or linked to research and/or innovation activities at the sending institution for at least six months (full-time equivalent) prior to the first period of secondment."
Additonally to this statement which leaves a lot of flexibility, the FAQ published by the EC state on p. 6: "For a RISE project it is of relevance that the seconded staff members are people involved in the research activities at the sending institutions. Hence, the engagement/linkage is related to the role of the staff member."
In more practical terms the key element to be taken into consideration is the contractual power of the sending organization with regards to the seconded persons. The existing link between the sending organisation and the seconded person should legally allow the sending organization to give instructions and supervise the seconded person in order to ensure compliance with RISE obligations (ex: full-time work, confidentiality, IPRs, use of resources, etc…see especially articles 6 and 32 of the GA). This is normally the case with employment contracts but, as in IRSES, other forms of contractual link are accepted provided they ensure to the organization the legal means mentioned above.
Please take note that undergraduate students cannot be seconded because they are still in a training and lack the expertise necessary to perform a research and innovation activities. Only students that have concluded the master degree can be seconded.