You can’t play a Minecraft server by “signing in to Roblox” because they’re separate games and separate accounts. If you mean Minecraft without signing in to a Microsoft/Xbox account, that’s only possible in limited cases, like local LAN play on the same Wi‑Fi or certain offline/single-player setups; normal online servers usually require a proper Minecraft account or an approved server setup.

What works

  • LAN play: If you and your friends are on the same Wi‑Fi, you can join a local world without signing in, depending on platform and settings.
  • Single-player/offline: Some versions let you play offline once the game is installed, but that does not mean public servers will work.
  • Custom/private servers: On some older or unofficial setups, people talk about “cracked” or no-account servers, but that’s unsupported and often buggy.

What does not work

  • You generally cannot join public Minecraft servers without an account in normal modern play. Public server access usually depends on authentication.
  • Roblox login is irrelevant to Minecraft servers, so there is no Roblox-based workaround for Minecraft multiplayer.

Safe next step

If your goal is just to play with friends, the cleanest options are:

  1. Use LAN on the same network.
  1. Use a proper Minecraft account for online servers.
  1. Set up a private world/server that everyone joins legitimately.

TL;DR: Roblox sign-in has nothing to do with Minecraft. For Minecraft servers, you usually need a Minecraft account unless you’re doing local LAN or another limited offline-friendly setup.