For our winds we generally are a pretty set group and usually are audition/invitation, but I do know that they cycle through subs pretty regularly. Strings tend to always be in need of numbers, hence why it’s open invitation for them. I absolutely think it’s worth reaching out via email to ask. I can DM you who to contact if interested. I don’t know if other reading orchestras, but there’s a healthy number of community orchestras in the surrounding suburbs.