RISE: What kind of contractual justification is required to participate in a RISE project in the case of collaborators of a NGO in an EU-MS/AC?

CONTINUATION OF THE QUESTION: The NGO in this case, has members who are people with disabilities and familiars, as well as voluntary collaborators. The relationship of all of them with the NGO is as members by paying an annual fee, and only some administrative staff perceives a salary.
Are the following examples eligible for a secondment?
- The president of the NGO, although she does this on a voluntary basis and has no salary for this work.
- A full professor of a University in this EU-MS/AC collaborates on a voluntary basis in the NGO as responsible of R&D in the NGO. He does not perceive any salary from the NGO. The relationship with the NGO is because he pays an annual membership fee to the NGO.
- A member of the NGO, a person with disabilities, who wants to collaborate in different tasks of the project and perform mobility.

The existing link between the sending organisation and the seconded person should provide to the sending organisation the legal and contractual means to impose to the seconded person compliance with RISE obligations. The seconded person must therefore work under the instructions and supervision of the sending organisation. This is normally the case with employment contracts but other forms of contractual relationship are accepted regardless of their nature provided they ensure to the sending organisation the legal means mentioned above.
Therefore, on the top of all other eligibility criteria applicable to RISE, the NGO must verify internally if the relationship with the three persons mentioned in your e-mail is enough to ensure the above legal means. Is the link with the NGO sufficient enough to allow the NGO to have contractual control/supervision over these persons?
Concerning the “professor of a Spanish university” please notice that in addition to the above the same professor will not be able to be seconded from both the NGO with which he/she collaborates and the University to which he/she is also linked.
(Answer provided by REA)