I can't say anything about this particular email you received, but I have received invitations to join the editorial board of two journals, which I declined. One was a very reputable journal, but one I did not publish in and to whose community of authors/readers I did not feel I belonged. (One would be right in asking why I was ever approached and I don't have an answer.) The other case is closer to the email you received. It was an Open Access journal and I declined because that is not a publishing model I support.
I believe that setting all practical considerations aside (whether one has time,...) the litmus test is whether one would be willing to publish in that journal. If the answer is negative, I think that you should decline.