“That’s content we definitely want on our platform,” AT&T’s chief executive, Randall Stephenson, said of “Friends” at an investor conference on Tuesday. “And clearly it’s important to Netflix as well.”

He added that the new agreement between the two is nonexclusive, meaning “Friends” is likely to appear on AT&T’s coming service by 2020. The sides are currently negotiating rates for after 2019 that would be significantly lower than $100 million, since the show would no longer stream exclusively on Netflix, the people with knowledge of the matter said. It’s also possible that “Friends” could leave Netflix after next year, they added.

To attract new streaming customers, AT&T will likely have to keep some of its shows and films off other services like Netflix and have them available only on AT&T’s offering. Disney, which plans to debut its own streaming service by the end of next year, will start pulling some of its films from Netflix after next year.

At AT&T’s analyst conference last month, John Stankey, the executive put in charge of WarnerMedia, said streaming players like Netflix were “going to see a pretty substantial structural shift” in the availability of content over the next 18 to 24 months.

Companies like Netflix “should expect their libraries are going to get a lot thinner,” he said.

Streaming services have anticipated this for years. In order to differentiate one offering from another, Netflix, Hulu and Amazon have stressed the importance of exclusive content. Netflix, for example, has spent billions building up original shows, including hits like “Stranger Things.”

“That’s going to make it exciting for us,” the Netflix chief executive, Reed Hastings, said on the company’s earnings call in October of the forthcoming competition. “It’s great for consumers. Incredible for producers. I mean there’s never been so much TV and movies being created around the world. So the game is on.”

The market will support two or three on-demand streaming services, including Netflix, Mr. Stephenson said on Tuesday, “and we want to be one of the two or three.”