Slack Technologies Inc. built up a $3.8 billion valuation and buzz as one of Silicon Valley’s hottest startups by getting millions of office workers to use its group messaging app. Now some of the tech industry’s biggest players want to be part of the conversation.

Microsoft Corp.’s this month unveiled its Teams workplace collaboration service, which came on the heels of Facebook Inc.’s launch last month of Workplace by Facebook. The tech giants’ moves put pressure on the San Francisco startup, whose user base is tiny by comparison.

...