What does each member contribute, and what do they take out?

He leads by example because he’s always doing a Zoom about something, contributing to the community and always asking you to figure out where you can too.

This occurred because of their improved financial situation, which resulted in higher contributions from the tenants.

Are they contributing to GFC now?

You mentioned you make too much to contribute to a Roth.

Many advanced hosts here can contribute or learn from each other, even at the next level. 

Instead of contributing to retirement at work, put that money into the next house.

Clearly define each person's roles, contributions, and responsibilities.

I look at it through the lens of: 1) If I contribute to a pre-tax account like a pre-tax 401k, Traditional IRA, SEP IRA, I get a tax deduction, but now I'm in a partnership with the IRS, but I don't know what I will have to pay them later on in life when I start pulling money out (that is because when you distribute you pay taxes based on your effective tax rate at that time, which can be unknown). 2) With Roth accounts, (Roth IRA/Solo 401k(k) with Roth component), I can get the taxes out of the way on the seed, so I don't have to pay on the crop. 

Ensuring you're both onboard from the start will contribute to a smoother process and a happier investing experience.