CustomGroup.java

package android.support.constraint;

import android.content.Context;
import android.os.Build;
import android.util.AttributeSet;
import android.view.View;
/**
 * Custom Group for ConstraintLayout that also supports alpha
 */
public class CustomGroup extends Group {
    public CustomGroup(Context context) {
        super(context);
    }

    public CustomGroup(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public CustomGroup(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    public void updatePreLayout(ConstraintLayout container) {
        int visibility = this.getVisibility();
        float elevation = 0.0F;
        if (Build.VERSION.SDK_INT >= 21) {
            elevation = this.getElevation();
        }

        for(int i = 0; i < this.mCount; ++i) {
            int id = this.mIds[i];
            View view = container.getViewById(id);

            if (view != null) {
                view.setVisibility(visibility);

                // getAlpha() came from the inherited View class, and can be defined in CustomGroup XML via android:alpha
                // or programmatically via setAlpha()
                view.setAlpha(getAlpha()); 

                if (elevation > 0.0F && Build.VERSION.SDK_INT >= 21) {
                    view.setElevation(elevation);
                }
            }
        }
    }
}

Usage:

<!-- Use whatever package name you use -->
<android.support.constraint.CustomGroup
            android:id="@+id/myGroup"
            android:alpha="0.2"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            app:constraint_referenced_ids="myview1,myview2"
            tools:ignore="MissingConstraints" />

or in code:

    myGroup.setAlpha(0.2f);