In other words, members who do not sign up will not be entitled to the benefit of the outcome if successful.
That isn't made clear either.
The original comment - that the letter could have been clearer - is viable.
Why not say:
"If you sign this letter, you will not be liable for any costs that come from this action".
and/or
"If you do not sign this letter, you will not receive any portion of what is awarded".
Or words to that effect?
The wording of the letter doesn't seem to be very enthusiastic either way.