Will I be sinful for participating in the establishment of a company with one person who occasionally drinks alcohol and another who finances his share with an interest-based loan?

It is not forbidden for you to partner with those mentioned, nor will you incur sin due to their personal transgressions such as drinking alcohol or borrowing with interest; as Allah Almighty says: "And no bearer of burdens will bear the burden of another." The prohibition of an interest-based loan pertains to the responsibility of its taker. However, scholars have disliked partnering with those who do not guard against the forbidden, such as a disbeliever or a Muslim who does not seek out the lawful, because they might involve their partner in unlawful dealings. Therefore, avoiding partnership with those who do not guard against the forbidden is more preferable and safer.