Is one who gathers people for travel with the intention of halal tourism and enjoying companionship sinful, despite knowing that some of them might commit sins such as drinking alcohol and fornication, even though his intention was not to help these people commit evil deeds?

If you do not predominantly suspect that some of your bad companions will commit indecencies if they travel with you, then there is no harm in helping them travel for permissible tourism. However, it is advisable not to accompany those who do not fear their Lord, as the Sharia has warned against accompanying evil people. The example of a good companion is like the musk seller, and a bad companion is like the bellows blower. The Prophet, peace and blessings be upon him, forbade accompanying non-believers and the impious, saying: "A person is on the religion of his intimate friend, so let one of you look at whom he befriends," and "Do not accompany anyone but a believer, and let no one eat your food except a pious person." An exception to this is accompanying them for the purpose of giving advice and inviting them to good, with the security of not being harmed. This is something that is commendable and may even be obligatory.