Many investors struggle with the idea of buying stocks at all-time highs, and their hesitation is understandable. But in these situations, try to remember that no one knows the future -- today's 52-week high could be the 52-week low a year from now.

With that in mind, shares of HubSpot (NYSE:HUBS) and Sea Limited (NYSE:SE) are up 73% and 76%, respectively, this year. And both recently hit all-time highs. Even so, you should consider adding a few shares of these growth stocks to your portfolio.

Here's why.

Investor looking pensive as he reads the newspaper.

Image source: Getty Images.

1. HubSpot: The pioneer of inbound marketing

HubSpot specializes in customer relationship management (CRM). Its software platform helps small businesses attract, engage, and retain customers; this includes tools that drive productivity across sales, customer service, marketing, and content management.

So what makes this company different? HubSpot pioneered the concept of inbound marketing. This approach differs from traditional outbound tactics, where viewers receive unsolicited ads as they browse the internet. HubSpot flips this model around, helping clients create content (i.e. social media, websites, blogs) to attract and engage leads, then convert those leads into customers.

Importantly, inbound marketing is much more precise, meaning clients pay less to acquire new customers. In fact, inbound leads are 61% cheaper than outbound leads, according to HubSpot. But if you're still not convinced, the company actually uses its own software to draw new clients to its platform. And HubSpot's solid financial results speak for themselves.

Metric

Q2 2018 (TTM)

Q2 2021 (TTM)

CAGR

Revenue

$441.4 million

$1.1 billion

34%

Free cash flow

$32.0 million

$144.5 million

65%

Source: Ycharts. TTM = trailing-12-months. CAGR = compound annual growth rate.

One of the most exciting things about this company is the market opportunity, which remains relatively unpenetrated. HubSpot currently has 121,000 customers, but management believes that figure could be as high as 3 million.

To that end, HubSpot continues to add new products to its CRM platform. Most recently, it launched Operations Hub, which enables clients to automatically sync customer data between business-critical applications. This gives sales and service teams access to the latest information, helping them provide a delightful customer experience.

Here's the bottom line: After strong gains through the first eight months of the year, HubSpot trades at 30 times sales, meaning the stock is quite pricey. Even so, the company has a big market opportunity, and (so far) an effective expansion strategy. That's why I think investors should consider buying this growth stock.

2. Sea Limited: The triple threat

Sea Limited is a holding company that runs three core business: the e-commerce marketplace Shopee, the video game platform Garena, and the fintech service SeaMoney. The company primarily operates in Southeast Asia and Taiwan, a region undergoing rapid economic growth, though Shopee has recently expanded into parts of Latin America and Europe.

Sea has established itself as an industry leader in each business. Shopee is the most visited online marketplace in the region, averaging twice as many monthly visitors as the next closest competitor, Alibaba's Lazada. And to reinforce that advantage, Shopee offers last-mile delivery through Shopee Xpress and payment processing through SeaMoney, both of which make its platform more convenient for sellers.

Not surprisingly, as Sea's online marketplace has scaled, so has its digital payments platform. Today, SeaMoney is a leading financial services provider in the region, and it's gaining traction with merchants off the Shopee marketplace. For example, consumers can use the mobile wallet to pay utility bills and order food for delivery.

Finally, Garena is the developer and publisher of Free Fire, the highest grossing mobile game in Southeast Asia, Latin America, and India. Unlike Sea's other businesses, Garena operates at a profit, generating cash that allows the company to invest aggressively in Shopee and SeaMoney.

Collectively, these advantages have helped Sea deliver triple-digit top line growth over the last three years, and the company is now free-cash-flow positive, suggesting that its business model is sustainable.

Metric

Q2 2018 (TTM)

Q2 2021 (TTM)

CAGR

Revenue

$557.5 million

$6.8 billion

130%

Free cash flow

($451.4 million)

$932.2 million

N/A

Source: Ycharts. TTM = trailing-12-months. CAGR = compound annual growth rate.

Here's the bottom line: Sea Limited operates in three high-growth industries, and the company has built a strong moat around its business. As a result, shares have skyrocketed 2,470% in the last three years. And despite trading at a pricey 26 times sales, I expect Sea Limited to be a long-term winner as e-commerce and digital payments continue to gain traction in Southeast Asia. That's why this growth stock still looks like a smart buy.

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.