I presume you mean an unpaid volunteer?
I have just started at Cosford, as an "explainer". One of the members of another group I work with is a guide, and he gave our group a tour of the museum. Despite 65+ years as an aviation "nut", plus 40 years in the business, I still learnt a lot. The biggest drawback seems to be the person who bones up on something esoteric, and then asks an obscure question. There's always one "smart a...". Used to happen with ATC cadets as well, so I suppose it comes with the territory. My advice is to see if the museum you are planning on joining has vacancies and take whatever is going, to get your feet under the table. It's perhaps easier then to swap roles.