How do I find who can help me obtain permission from RP to grant permission to recruit for doctoral dissertation?

SFC Melvin Brandenburg One of our Senior RallyPoint Administrators will be able to answer your question.

SSG Carlos Madden, LTC Kevin B.

Excellent topic for research SFC Melvin Brandenburg! Best of luck with Your endeavors!

Best of luck

RP is pretty limited. I don't know your topic, but you don't have a better resource?

This has already been pretty much answered, but just to throw it out there, Remember that Retention Control Point (RCP) is a thing, at least in the Army.
The only reason I say that is you mentioned 20 v. 30. But for SSG there is no option past 20, for SFC, it is 20 v. 24, and for MSG/1SG (non-promotable) it is 20 v. 26 (at least in the Army, not sure how the sister services do it). And for Officers it is different entirely - and there is pretty much never a 10 year window. If you are looking at 20 v. 30, you are looking strictly at SGMs and CSMs, which will GREATLY reduce your sample pool.

The question really becomes how do you want to contact your subjects. Open messages to RP is usually illadvisored both by their ToS and your dissertation parameters. As someone that is also conducting his dissertation on leadership and retention, there are very strict guidelines on how you contact subjects and in what manner. Be sure to speak to your dissertation advisor as well as your IRB board to ensure RP is an acceptable means.
When thinking about your sample, contacting specific people could cause a problem in your sampling and create some bias or errors. Have you established your literature review and method and methodology? What is your parameters for sampling?