Co-ownership
I am closing on a vacation / investment beach condo within next 60 days and already have approved mtg. in name of myself and spouse. Now we have another family interested in 50% ownership of condo. However, value has appreciated considerably since entering into pre-construction contract. I am considering selling 50% interest at the current FMV. However, it's too late in game to put property into LLC and get investor mtg. If both families sign on to the mortgage, can we execute a side agreement outlining issues of the property such as what happens if one party does not stay current on their share of mortgage, etc. ....I'm a banker and understand risk of other side not paying....but I have capicity to keep mtg. current as long as I'm protected by diluting other family's interest in the property....is this possible...or should I delay closing to put property into LLC and find investor mortgage?? 
Why is it too late to buy in a structure?
if it's joint tenants, make sure you have a very tight co-tenancy agreement including things such as
death
buy/sell
right of 1st refusal
divorce
etc,
Might I suggest the following:
1. Close on the property with you and your wife taking title as "tenant in common". 2. Sell 50% at current FMV to your friends with a Contract for Deed. If they walk, you still own the property. They will take ownership as tenant in common also after the full amount of the COD is fulfilled. 3. Form your LLC and quit claim the property into the LLC. Holding the Property in Tenant in Common, you can hold 50/50% for your wife and you both selling 50% of each of your shares, or you own 75% and your wife owns 25%. (You sell 2/3 of your shares to the other family.) Note: Joint Tenancy can only acheived by taking title at the same time. You cannot ADD another owner into the Joint Tenancy later. They can only take the deed as Tenant in Common. You get your property, your tax deduction on the mortgage, and RE taxes, limit your legal risks (w/LLC) and the other family recieves the deductions too. If they leave, you still own the property, and keep the $ from their payments to you.
The COD must be recorded, but the mortgagee will not review the records, so you should be fine. You will have to record the LLC's ownership too.