Guess Who’s Using Your Netflix Account? You Might Be Surprised.

“That’s ridiculous,” Mr. Ringlein, a venture-capital investor from Brooklyn, recalled thinking when he found out. “They’ve just been watching the whole time.”

Netflix estimates some 100 million people—on top of its 230 million paying subscribers—are watching the service using somebody else’s account. The streaming giant has mostly shrugged off password sharing but plans to soon start cracking down, as subscriber growth has slowed.

Getting subscribers themselves to boot the moochers off their accounts is a way to nip off the lowest-hanging fruit. Netflix is hoping some of the people who are kicked off will sign up for their own accounts.

The company recently launched a tool that lets users see where and when an account has been accessed. The results can be as surprising as a plot twist in a Netflix series.

Mr. Ringlein said he found out other people were using his account when he opened the new dashboard. He used it to disconnect his account from his old Tesla and the Airbnb, whose users had streamed shows and films he said he’d never watch, including the Lindsay Lohan Netflix film “Falling for Christmas” and an episode of “CoComelon,” a popular nursery-rhyme show.

Luke Stronach, a 25-year-old charity worker in Glasgow, knew something was off when Netflix recommended a show about sex to him based on his viewing patterns, despite the fact that he said he mostly watches shows like “The Crown” and “It’s Always Sunny In Philadelphia.”

He checked his viewing history, and found someone had used his account to watch “How to Build a Sex Room.”

He also noticed his account name had been changed to “Flat 13A.” He thinks the new residents of the Strathclyde University dorm where he once lived—and watched Netflix on a shared TV set—came up with the new name.

“Now that I know, it’s quite funny,” Mr. Stronach said. He renamed his account and changed his password.

In the coming weeks, Netflix will expand the pruning effort and start forcing users who share their account with people outside their households to pay a bit extra to continue doing so. Prices aren’t yet detailed, and other restrictions on total number of extra users allowed and their locations might also be in the works. Only people living under the same roof as the account holder are safe.

Netflix late last year introduced a feature that lets people transfer a particular user’s profile, which includes a viewing history and watch list, to a new account.

“Our job is to give them a little bit of a nudge, and to create features that make transitioning to their own account easy and simple,” said

Greg Peters,

Netflix’s co-chief executive. The company said the new features give subscribers more control over their accounts.

The changes threaten to unravel many users’ convoluted password-sharing agreements among friends and family who don’t live together.

Estefany Dominguez, a 23-year-old from the Bronx, pays for Netflix and Hulu, while her sister pays for HBO Max. They share HBO Max with Ms. Dominguez’s boyfriend, who lets them both use his

Disney

+ account.

She said she would think hard about whether to pay more to continue sharing Netflix with her sister. “She’s a broke college student,” she said of her sister. “I’m inclined to say you deserve a little entertainment.”

Mitchell Condie, a 27-year-old Las Vegas resident, watches Netflix using his sister’s account, while he gives her access to his HBO Max account. Both share those accounts with their parents, who in turn pay for Hulu for the entire family.

The agreement works seamlessly, Mr. Condie said, because the family members in the pact—who live in Utah—years ago agreed that each person would pay for the premium tier of their respective streaming services so that everyone could watch simultaneously without issue.

Some subscribers can’t wait for the crackdown to begin.

George Anghel, a 32-year-old in London, years ago shared his Netflix account with a couple of friends and his father, who then shared it with Mr. Anghel’s brother. He said he now often finds himself unable to watch Netflix because too many other people are already doing so. When he recently checked the new Netflix tool showing where and when his account was accessed, he saw six devices were logged in beyond his own.

“No wonder we can’t watch,” he said.

Mr. Anghel said he recently turned down another friend who asked for his Netflix credentials but is reluctant to kick out the family members and close friends already using it.

“Someone has to come and say no,” Mr. Anghel said. “I think Netflix should say it, because I’m paying them.”

Write to Sarah Krouse at [email protected]