Have a look at the Facebook or webpages for the orchestra(s), they will usually indicate if an audition is required, or you can get in touch.
As for practising an instrument in halls, in my experience, that's never been massively easy unless you're a music student, as practice rooms are few and far between and constantly booked up, and you're not often allowed to practise in your room. If you have private accommodation, it's a bit easier.