Group Reviews are BP activity items. BP does not allow non-members to post activity items to groups. You’d have to dig into BP to see where this restriction is upheld, and find a way to work around it based on that. (In other words, you’ll have to look beyond BP Group Reviews.)
thanks for ur answer.. i will try to find a way..
Actually i think other people may be in the same situation : for example if you have a website dedicated to soccer and if all your group represent several teams (private groups) it would be interesting to rate or review other teams even if you are not one of its members… And i also suppose that BP reviews would not be interesting in this case cause all players would rate 5 stars and would write good reviews about their own team… 
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