Welcome to the Zoom Community, @MonicaW.
Merging the account would associate your account with the organization. I agree that this is not suitable for lots of volunteers and their organization.
In your case, I recommend that you do not accept the invitation, and that you get a separate email address to access your "volunteer" account. For example, set up "MonicaWVolunteerABC" (where VolunteerABC is some sort of description of what/where you're volunteering) on gmail, then forward all of that email to your primary email address. (Alternatively, set up your phone and computer to access both email accounts. I keep my volunteer Email separate from my personal and business email for a variety of reasons -- some of them due to legal discoverability of certain volunteer work, like board of directors, etc. you might think it's a little more work keeping track of multiple email accounts, but I find it helps me focus!)