First names are quite hard for some of these characters. Allowing either first or last name would make it easier for some characters, but without a clue, I'd never remember the extremely minor characters.
tonks, bill, charlie and fleur were not old enough for the first order i think they were still in school yes i think they were and james and lily were dead for the second one
Tonks, Bill, Charlie and Fleur had graduated, Fleur was a champion in the Triwizard Tournament, for only 17-year-olds or older, Fleur graduated when Harry was in year 4.
I think they didn't want to formally be members and do active things that could be dangerous, but they of course agreed with the goal of the Order and were willing to help out on occasion with less overt stuff.