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<div class="row">
    <!--Nav Bar -->
    <nav class="col-xs-3">
        <ul id="sidebar" class="fixed">
            <li>
                <a href="#GroupA">Group A</a>
                <ul>
                    <li><a href="#GroupASub1">Sub-Group 1</a></li>
                    <li><a href="#GroupASub2">Sub-Group 2</a></li>
                </ul>
            </li>
            <li>
                <a href="#GroupB">Group B</a>
                <ul>
                    <li><a href="#GroupBSub1">Sub-Group 1</a></li>
                    <li><a href="#GroupBSub2">Sub-Group 2</a></li>
                </ul>
            </li>
            <li>
                <a href="#GroupC">Group C</a>
                <ul>
                    <li><a href="#GroupCSub1">Sub-Group 1</a></li>
                    <li><a href="#GroupCSub2">Sub-Group 2</a></li>
                </ul>
            </li>
        </ul>
    </nav>
    <!--Main Content -->
    <div class="col-xs-9">
        <section id="GroupA" class="group">
            <h3>Group A</h3>
            <div id="GroupASub1" class="subgroup">
                <h4>Group A Sub 1</h4>
            </div>
            <div id="GroupASub2" class="subgroup">
                <h4>Group A Sub 2</h4>
            </div>
        </section>
        <section id="GroupB" class="group">
            <h3>Group B</h3>
            <div id="GroupBSub1" class="subgroup">
                <h4>Group B Sub 1</h4>
            </div>
            <div id="GroupBSub2" class="subgroup">
                <h4>Group B Sub 2</h4>
            </div>
        </section>
        <section id="GroupC" class="group">
            <h3>Group C</h3>
            <div id="GroupCSub1" class="subgroup">
                <h4>Group C Sub 1</h4>
            </div>
            <div id="GroupCSub2" class="subgroup">
                <h4>Group C Sub 2</h4>
            </div>
        </section>    
    </div>
</div>

.group {
    background: yellow;
    width: 200px;
    height: 500px;
}
.group .subgroup {
    background: orange;
    width: 150px;
    height: 200px;
}
.fixed {
    position: fixed;
}