The above two replies are too out of date from the most recent version of the main viewer and it would appear that if you add someone as a member to your group that you inteded on giving them a different role, you are not given the option to change their role while they are still available within your group...and you might have to temporarily kick ("Eject") them from your group and re-assign their title from the get-go, because it appears that it is only within that window of early group-joining invite stage, that you are able to cement them as that role or not. Good luck changing their roles any further, after that (if you are struggling with the same barriers I am and your OS is "too old" to run Firestorm or "not Windows enough" to run any other viewers).
An annoying hiccup, but it appears to be the only way, at this time. No idea why no one else has bothered to update this thread, since 2015.
Happy to hear & consider any other available/accessible options out there!
Thanks,
--'M.C.