Scenario 1 (most likely): They are just some people tryna scare you and they don't know your ip address.
Scenario 2: They are an advanced techy who somehow found your ip address by either injecting code into the game host which would violate the terms of use which means they could be banned from roblox OR they used something like netstat integrated with their browser to find the people connected to their game OR they used some other method tooooo advanced for me to know about.
Contradicting to what BraceletGal123 said, if someone who knows what they are doing finds your IP address that would be VERY bad. They could: Find your exact location and hunt u down OR ping (send a request to your computer asking for a response) your computer with unusually large amounts of data and overload your wifi network and block all other people who want to connect to you which would shut down your wifi OR some other method I don't know.
Final message: Whoever said that to you was probably just lying to make u scared or angry. Don't be worried but if your WiFi starts acting strange and shutting down then I would tell your parents about what is happening. My best bet is that they were lying and messin around with you to make u angry. DONT WORRY!
Side note for people who just-want-to-know: You have two kinds of IP addresses. There is your local address that starts with something like 192.168.x.x that the people in your home uses to communicate with you on their computers. The other kind of IP address is your public IP which is what you DON'T want to give to other people. Your local IP address is just something used in your home and everyone else has an address similar or the same to yours if they live outside of your wifi network.
PS. I have the CompTIA A+ cert so what I said might not make a ton of sense. If you want more clarification on what I said, you can just ask. Thanks! (KS If you are confused and think I am an adult just tell me and I will clarify. Thanks!)