We'll need to make sure this is legitimate, is there word from the official Monero development team?
It's legitimate in the sense that is it definitely engaged in communications with the core team, and I have no reason to doubt the commitment to donate 50% of membership fees to the core team for devleopment. Further some of the core team may also be members of this group, though such membership would be as individuals and not representing the core team (nor vice versa).
However, it is also an independent group with its own charter and direction, so you will need to use your own judgement as to whether it is something you want to support. The option to donate directly to the core team and decline to participate in this new group remains.
What is the relation of MEW and the core team?
Core team exists by their own right, chooses their members, and they have defined their role is to develop the Monero software.
MEW exists as a voluntary group of people who own XMR, and is bound to accept more members based on this criterion. Because the core team members also own XMR, they are eligible and welcome to join MEW, and some have been interested in doing so.
MEW will concentrate on the peripheral/economic aspects of the coin infrastructure, and the people in the community, leaving the core team more time and resources to concentrate on the development. MEW will also support the donation address.
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Shouldnt the devs decide the priorities? I mean they are the once who must donate their time to implement features.
The core team retains complete control with how to develop the coin, Monero (XMR). The role of MEW is to give them valuable information on the community's wishes, but we cannot force even our own members to do anything, much less the devs who - as a collective - are not even members. In a similar way we may issue suggestions to whomever, but they are not bound to follow the suggestions, and we will employ great care to not meddle in others' affairs, and also expect nobody to tell us how we should make use of our XMR privacy coin.