See "How do I confirm authorship of my publications to add them to my profile?" in https://help.researchgate.net/hc/en-us/articles/14293005132305-How-to-add-research for instructions.
This paper is already at ResearchGate (https://www.researchgate.net/publication/381339452), but without any authors because in the original the authors (i.e. members of the working group) are listed in the acknowledgements. Therefore is may be difficult or impossible to claim authorship.
I think you can try to claim authorship. See for example Article Burden of disease scenarios for 204 countries and territorie...
where something comparable is going on.
I recall that I saw something similar in various medical papers where hundreds of collaborators (in hospitals) were mentioned in the end of the paper and these collaborators were able to claim authorship as well.