You have to be sure it’s truly structured as debt so it isn’t re-characterized as a non-qualifying equity contribution, but assuming you use a reasonable interest rate and have an amortization schedule for it, you could loan money to the LLC as you describe. 

I may also do a "fund" for something, such as a down payment for a house or something if people would rather contribute ti that. 

It would be like making 'excessive contributions'.

Thanks again to all who contributed,BRRRRzerkeley Greg

If you break from the 50/50 model, you will need an agreement regarding partners' contributions, distributions, roles and conditions upon termination of the partnership.   

In addition, if you are self-employed with no full-time employees you may wish to consider opening a Solo 401k instead of a self-directed IRA as it has several advantages over an IRA LLC such as much higher contribution limits, direct checkbook control (i.e. no need to have the account at a specialty trust company), ability to take a 401k loan, exclusion from unrelated debt finance income tax with respect to investment in real estate acquired with non-recourse financing, etc.In addition, please note if you purchase debt-financed real estate with your IRA, unrelated debt finance income tax should apply to the income attributable to debt-financed real estate held by your IRA.

Roommates keep thinking they're only responsible for half (if two only) of the rent.You need to disabuse them of this foolish notion.

However, adding in a daughter to the LLC will require a separate tax filing>First you have to determine what the purpose of creating the LLC whether making all the contributions will achieve that will not facing any negative consequences.

My friend is going to contribute funds for rehab and cap ex and rent the other half.

We, equity partners, only got paid approx 20% of initial contribution.