Oggin and TS,
My interpretation of that paragraph on page 7 of the orange book is that you have one option to join and one further option to re-join if you temporarily opt out and join a private scheme.
Your one option to join is now.
But thats only my opinion, although I'm quite sure that the OTT would have made it quite clear that you can defer your decision to any future time while you remain in the Services. I believe it doesn't offer deferrment (I haven't received mine yet, so standing on thinnish ice), but Ginseng will probably know more about this.
PS. I Just found the answer half way down the left hand column on Page 1 of the OTT Booklet, available at the MOD pensions website,
here
It is quite clear that you have only one opportunity to join the new scheme. The orange book paragrah can be misinterpreted