The most significant difference is that a Rental Agreement is signed between tenants and the landlord renting them a property, while a Roommate Agreement is signed between people who intend to live together. Typically, a Roommate Agreement can be customized more than a Rental Agreement.
No, you’re not required to notarize a Roommate Agreement. For additional security, you can do so, but inviting two or more witnesses, although that’s not a requirement either.
If a new person wants to move in, you might first need permission from your landlord. That requires updating your Residential Lease Agreement, and it may be a good idea to create a brand-new Roommate Agreement afterward.