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I have contacted jackjack and also I am looking for the developer of coinig.
Thanks.
just want to check can greenbits sign a message?
if yes, how

You can't.
Correct, this is true for all multi sig addresses or services that use multi sig. The problem is that there is no standard how a signature would be done with more than one key[1].
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so...is it better to change an wallet with sign message funtion?
as I may want to sell the account in the future.
You dont have to use the same wallet for signing as you use for spending. You can create a wallet just for signing messages.
[1]
https://www.reddit.com/r/Bitcoin/comments/2k3t9c/can_i_sign_a_message_using_greenaddress/clhmbq2