Investing in an index fund that tracks the S&P 500 is a surprisingly simple way to grow your money over the long haul. It's inherently low-maintenance, low-risk, and low-stress. Of course, it can also be a little boring, and the returns are, by definition, average. While returns are more important than flair when it comes to investing, it doesn't hurt to inject a little above-average growth into your portfolio.

With that in mind, we recently asked three contributors at The Motley Fool for a growth stock that may not be on the radar of most investors. Here's why they chose bioprocess leader Repligen (NASDAQ:RGEN), DocuSign (NASDAQ:DOCU), and home goods supplier Wayfair (NYSE:W).

A technician servicing a stainless steel bioreactor used in drug manufacturing.

Image source: Getty Images.

Powering the rise of biopharma

Maxx Chatsko (Repligen): Most investors have probably never heard of Repligen, but the stock has quietly delivered a 10-year return of 1,570% by helping to power the rise of biopharma. The bioprocess leader supplies the tools needed to safely and efficiently manufacture biologic drugs, such as monoclonal antibodies and (soon) cellular medicines, at volumes suitable for clinical trials or drug franchises reeling in billions of dollars per year in revenue. It doesn't develop drug candidates.

Management has done an excellent job acquiring interesting technologies that fit into the company's focus on bioprocess engineering, integrating the assets into existing operations and investing in their continued development. The ability to continuously respond to customer needs and stay one step ahead of the fast-moving biopharma field has positioned Repligen on an envious growth trajectory.

The business reported first-half 2019 revenue of $131 million, representing year-over-year growth of 42%. Total costs and operating expenses grew just 32% in that span, which allowed a growing share of revenue to trickle down the income statement as profit. Repligen grew operating income 117% in the year-over-year comparison. 

That said, updated full-year 2019 guidance suggests that the business will wade through a period of higher expenses in the second half of the year as it integrates its latest acquisition and pays more in taxes than previously expected. Full-year 2019 diluted earnings per share (EPS) is expected to be about $0.36, which isn't much different from first-half EPS of $0.34. Nonetheless, that will do little to knock Repligen off track. The company ended June with $209 million in cash and raised over $200 million more through capital raises after the end of Q2. 

Investors looking for growth will definitely have to pay a premium, and may want to wait for a pullback in the stock price, but the business has plenty of growth ahead of it in the next few years.

A potted plant trimmed in the shape of an arrow pointing up and to the right.

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The e-signature leader

Brian Feroldi (DocuSign): Technological advancements have made many of our day-to-day activities more efficient. However, the majority of agreements that are completed today still require a pen to put down a signature. How come this centuries-old process hasn't been disrupted by technology? That's a question that DocuSign is on a mission to answer.

DocuSign provides cloud-based software that allows business agreements to take place online. The number of use cases for DocuSign's services is vast: Businesses are currently using the product to help with invoices, sales arrangements, legal contracts, nondisclosure agreements, and 70 other documents.

For investors, DocuSign boasts many traits that make for an attractive investment. The company hauls in high-margin recurring revenue and is growing fast. It's already producing adjusted profits and free cash flow. The company gets great reviews from employees and has a beloved leader at the helm. To top it all off, it's the top dog in a market that could eventually be worth $25 billion. That's a huge number when compared to the $920 million that it is expected to haul in this year.

More recently, DocuSign's stock has come under pressure because of a deceleration in its billings. While that trend is worth keeping an eye on, the recent sell-off looks overblown. Granted, shares aren't exactly cheap -- the stock is currently trading for more than 10 times sales and 120 times next year's earnings estimates -- but I think there's so much to like about this business that the premium is worth paying to become a shareholder.

An empty office with exposed wood beams and modern furniture.

Image source: Getty Images.

Betting on profits to come

Demitri Kalogeropoulos (Wayfair): When a company makes a habit out of blowing past its own aggressive sales growth targets, it's worth your attention. Wayfair fits that description perfectly, with its recent second-quarter results marking a fifth consecutive quarter of surprisingly strong sales gains. Revenue improved 42%, in fact, to $2.3 billion, compared to the early-May target executives issued of 35%. In addition to the market-beating growth, Wayfair notched impressive growth in key engagement metrics like repeat order volume and average order spending.

Sure, Wayfair is far from generating real profits. It is set to drive deeper into the red in fiscal 2019, after all. Yet there are good reasons to believe these losses won't stick around for long.

First, the home furnishings specialist's advertising spending is drifting lower as a percentage of sales, implying that it is building an efficient, defensible market position. Second, most of the recent losses can be tied to hiring sprees that are its aggressive growth both in the U.S. and in new international markets like the U.K. Once these initiatives run their course, Wayfair will be ready to allow more of its cash to flow toward the bottom line rather than into areas like the supply chain and engineering. That potential seems more likely with each passing quarter of sales growth at 30% -- or more.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.