WOLips "finds" models just like NSBundle does (i.e. hunt and auto-include all the .eomodeld's). The problem with that behavior is that some people actually have multiple versions of the same model (they have OracleModel, FrontBaseModel, etc. instead of using prototypes ++). But EOModelGroup's namespace has to be unique on model name and entity name, so that blows up with NSBundle-style "find everything".
EOModelGroup's in WOLips, consequently, let you define an explicit list of models to load into Entity Modeler. Then rather than open your model with Entity Modeler, you open your .eomodelgroup with Entity Modeler and it will avoid the conflict.
++ this is a really, profoundly bad idea. Use prototypes.