Praise be to Allaah.
You should advise your friends to repent to Allah, may He be
exalted, from what they have done, because dealing with riba, whether by
lending or borrowing, is a major sin, and there are warnings concerning riba
that were not issued concerning any other sins, including the curse on the
one who does it, the one who writes it down and the two who witness it. We
ask Allah to keep us safe and sound.
Money acquired through a riba-based loan -- even though it is
haraam and abhorrent -- becomes one’s property according to the correct
scholarly view; this is the view of the Hanafis and Hanbalis, and is one
opinion of the Shaafa‘is. In other words, the one who takes out a riba-based
loan becomes the owner of the money that he borrowed. Based on that, it is
valid to start a project with it, despite the sin of riba.
See: al-Manfa‘ah fi’l-Qarad by ‘Abdullah ibn Muhammad
al-‘Umraani, p. 245-254
According to this opinion, it is permissible for you to join
them, whilst taking care to avoid joining them in any haraam transaction.
But if you can avoid joining your friends in this money
altogether, that will be better and will be more on the safe side for you,
and it will be more effective in objecting to their entering into the haraam
And Allah knows best.