For example, I had a 3 bedroom unit in which 2 brothers rented and they wanted to have a co-worker move in as well.

If a third party is willing to be responsible in case of tenant default, then put them as a co-signer on the lease/rental agreement, not as a co-tenant.

So, with this disclosure – I am a co-founder and CFO of SureDeposit, please note:1) We do indeed look for a 1,000 unit minimum size for client participation, although we make exceptions from time to time;2) Loss limits are an integral part of the program (in reply to DeuceSevenOff).

Make certain you speak with past landlords, including the one who evicted.If your lease provides for a guarantor (as opposed to a co-tenant), you can have them sign the regular lease.  

In calculating my debt, they are allocating me the full amount of the mortgage payment for the currently existing loan even though I am a co-borrower.  

a co-signer's piece of collateral, etc.4.  

If so, you are subjecting yourself to a COE complaint.Have a nice day.PG

*I think a co-brokerage on a deal is more difficult because of potential liability.

and if they don't, then I ask for a co-signer? 

Again, I will pay everything myself (small mortgage, insurance and taxes) and technically don't need a co signer but it seems silly not to take at least consider this option.