Allow specifying which Project Groups have access to Environments
I have different applications that deploy to different sets of servers. I could create unique environments to handle the separation but it could get ugly pretty quick. If a project group was tied to a set of environments it would make it easier to know what environments apply to that project and prevent me from selecting the wrong environment to deploy to.

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Kevin Callahan commented
We have several project group on our server and we actually create environments per project group to make the mapping clean - it is ugly, but IMHO less ugly than having application1 servers in the same pile as application2.
I think you could also fix this by moving the mapping of servers to environments into the project or project group.