Lenders always assume the worst case scenario, that is if the borrower defaults and cant pay the loan then you as the guarantee will be deemed responsible for the repayments.
Other lenders will factor the entire loan into the equation when it comes to calculating your serviceability.
I think what Rolf was getting at was are you signing a Director's Guarantee as director of your own company for borrowings in your own personal name/entity or giving a guarantee to a third party who is not yourself.
Guaranteeing for your own Trust/company is ok. Guaranteeing for a parent/sibling/friend will bite you. Yes it will affect your borrowing capacity negatively.