When using grouping it took some digging around in code to figure out why it wasn't working. With SOLR 3.x which is what Acquia use you can only group off of string and boolean field types.

Would suggest the following edit to the project page:
Enable and configure the "Grouping" processor of an Search API index and configure the fields to group on, note that if using SOLR 3.x grouping will only work on string or boolean fields.

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heylookalive created an issue.