So I did get this working. In my case, the problem was passing a proper path name without "file://" at the beginning. Using url.absolutePath does this, so you need to strip it off when converting URL's. Here is some sample code:

// Get a URL to the shared file location
func getSharedFilePath(appGroup:String)->URL? {

    if let directoryPath = FileManager().containerURL(forSecurityApplicationGroupIdentifier: appGroup) {
        return directoryPath.appendingPathComponent(sharedFilename)
    } else {
        return nil
    }
}

public func writeToSharedFile(version:Int)->String {

    guard let fileURL = getSharedFilePath(appGroup:applicationGroup) else {
        return("Error getting shared file path")
    }

    guard !FileManager.default.fileExists(atPath: fileURL.path) else {
        return("File already exists")
    }

    // get path to file in app bundle
    guard let tpkFile = Bundle.main.url(forResource: appBundleFilename, withExtension: "tpk") else {
        return("Error finding file in app bundle")
    }

    // read file from app bundle to data variable
    var data:Data? = nil
    do {
        data = try Data(contentsOf: tpkFile)
    } catch {
        print("Error saving file")
    }

    // write data variable to file in file system
    do {
        try data?.write(to: fileURL)
        print("Success: shared file saved: \(fileURL)")
    } catch {
        print("Error saving shared file")
    }

    return("ok")
}

public func readFromSharedFile()->String {

    guard let url = getSharedFilePath(appGroup:applicationGroup) else {
        return("Error getting shared file path")
    }

    // read file from file system to data variable
    var fullPath = url.absoluteString
    // strip off "file://"
    fullPath = fullPath.substring(from: fullPath.index(fullPath.startIndex, offsetBy: 7))
    let basemap = AGSLocalTiledLayer(path: fullPath)    //old syntax

    if let error = basemap?.error {
        print("Read shared file error: " + error.localizedDescription)
    } else {
        print("Read shared file loaded OK")
    }

    return("data file read successfully")
}