Give an inch, take a mile.
I'm in a similar situation with my coworker and giving her rides home. The past two times, she invited people. I'm not a taxi driver. I'm giving her a bullshit excuse today, and in the future I'm not doing carpool shit unless I become friends with the coworker, which has happened in the past too at a different job and ended more amicably. I've been giving this woman rides because she complained to the boss that her uber fare was too expensive and he suggested asking me to help. I checked and she can learn how to use the bus, but she said she doesn't know how. Not my problem anymore.