Edit in JSFiddle

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

/* sidebar */
.bs-docs-sidebar {
    padding-left: 20px;
    margin-top: 20px;
    margin-bottom: 20px;
}

/* all links */
.bs-docs-sidebar .nav>li>a {
    color: #999;
    border-left: 2px solid transparent;
    padding: 4px 20px;
    font-size: 13px;
    font-weight: 400;
}

/* nested links */
.bs-docs-sidebar .nav .nav>li>a {
    padding-top: 1px;
    padding-bottom: 1px;
    padding-left: 30px;
    font-size: 12px;
}

/* active & hover links */
.bs-docs-sidebar .nav>.active>a, 
.bs-docs-sidebar .nav>li>a:hover, 
.bs-docs-sidebar .nav>li>a:focus {
    color: #563d7c;                 
    text-decoration: none;          
    background-color: transparent;  
    border-left-color: #563d7c; 
}
/* all active links */
.bs-docs-sidebar .nav>.active>a, 
.bs-docs-sidebar .nav>.active:hover>a,
.bs-docs-sidebar .nav>.active:focus>a {
    font-weight: 700;
}
/* nested active links */
.bs-docs-sidebar .nav .nav>.active>a, 
.bs-docs-sidebar .nav .nav>.active:hover>a,
.bs-docs-sidebar .nav .nav>.active:focus>a {
    font-weight: 500;
}

/* hide inactive nested list */
.bs-docs-sidebar .nav ul.nav {
    display: none;           
}
/* show active nested list */
.bs-docs-sidebar .nav>.active>ul.nav {
    display: block;           
}

/*Header formatting */
.MainContent {
    margin-top: 50px;
}
.group, .subgroup {
    padding-top: 50px;
    margin-top: -50px;
}

$('body').scrollspy({
    target: '.bs-docs-sidebar',
    offset: 40
});

<!-- Top Navbar -->
<div class="navbar navbar-inverse navbar-fixed-top">
    <div class="container">
        <div class="navbar-header">
            <button type="button" class="navbar-toggle" data-toggle="collapse" data-target=".navbar-collapse">
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
            </button> <a class="navbar-brand" href="#">Bootstrap 3.0 Skeleton</a>

        </div>
        <div class="collapse navbar-collapse">
            <ul class="nav navbar-nav">
                <li class="active"><a href="#">Home</a>
                </li>
                <li><a href="#about">About</a>
                </li>
                <li><a href="#contact">Contact</a>
                </li>
            </ul>
        </div>
        <!--/.nav-collapse -->
    </div>
</div>

<!-- Main Content -->
<div class="row MainContent">
    <!--Side Nav Bar -->
    <nav class="col-xs-3 bs-docs-sidebar">
        <ul id="sidebar" class="nav nav-stacked fixed">
            <li>
                <a href="#GroupA">Group A</a>
                <ul class="nav nav-stacked">
                    <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 class="nav nav-stacked">
                    <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 class="nav nav-stacked">
                    <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>