I'm looking for some input for best strategies to either partner individually with each investor and form an LLC to hold the property or, create one or more LLC's with multiple members with all of us working together. 

Assuming everything checks out with the sister (references/income/credit check) I had planned to get another new deposit from her, return his deposit (less any damages due to him), and create a new lease with my original tenant and her sister.

Get a much knowledge as you can, dont be afraid to start a deal, find a mentor, assemble a team that works for you, and most of all create a niche in real estate you enjoy...Lee

Are you looking to buy turn-key properties or create these opportunities yourself?

This is one of the reasons I created this thread is to validate before taking any action.

Another way I was advised was to create a rental agreement between myself & my unmarried partner and the LLC, then pay the “rent” from a personal account to the business checking account.

Obviously, not all brokers are created equal.

I think that you have a specific competitive advantage over casual investors in that you work full time in the field and can create opportunities based on your knowledge disparity vs the general public and should be looking to capitalize on that especially if you don't have millions in the bank.I took a similar path and it has been extremely beneficial for both facets of my business. 

@Jay Helms thanks for asking, I want to invest to create the long term cash flow so that I can eventually quite my full time job and work on my passion.

I agree with Jeffrey's post, it will create a double layer if you run into litigation issues in the future.