Answer: This isn'tdirectly addressed in the Bill, but since it is the space that is being rented, and the home with tenants have been there three years, I don't view this as a new tenancy. As I see it, rent increases going forward are for a home that has been on the space to the same residents for three-years; bringing in an additional tenant who has been there less than one year is not a factor, since the base rent is for the home on the space, and is not measured on a per tenant basis. But this could be subject to differing interpretations.
By the way, I do not view the first-year freeze on rent increases as particularly harsh, since the landlord can negotiate the first year's rent before allowing the tenant to come in. For example, if current residents are paying $400/month for space rent, but you are planning a rent increase - or have already issued one, for $40 dollars, you would presumably accept the new tenant at $440 - thus making the first-year freeze irrelevant.
On month-to-month tenancies, there is no "cap" on the amount of the initial base rent - it may start at whatever level the landlord and tenant agree upon. However, it cannot be increased thereafter during the first year of the tenancy. For fixed term tenancies, i.e. leases, landlords generally have their rents established through a formula contained in the written agreement. SB 608 only modified ORS 90.600, which governs periodic tenancies, such as month-to-month tenancies.