Volunteers can be a huge blessing, but of course, they need to be trusted, as well as skilled. We have had volunteers who were retired but volunteered in IT as if it were a part-time job. We have some who have retired from our IT department, but still help us on occasion when we have a project requiring more hands. We’ve also used unpaid interns. Our permission levels are such that we can grant access to various resources that are needed, while not allowing others. Of course, that’s largely on the PC side. For Macs, giving a volunteer an administrative login might be problematic, since changing all the admin passwords on all the Macs could be time-consuming.
Volunteers can be outstanding for producing documentation, doing repetitious tasks (such as installing updates, setting up new hardware for deployment, etc.), doing physical tasks (such as moving equipment from one location to another), taking phone calls to the Help Desk (whether they provide support or take messages depends on their level of experience and comfort in your environment).
If they have advanced experience in systems, networking, hardware/software support, security, or whatever, and they have expressed an interest in helping, it might be foolish NOT to let them help!
As with an employee, any volunteer who wants to come into your shop and do things their own way, or touches systems that they have not been trained on, or makes changes without notifying people, or changes settings without changing them back, might be more of a hindrance than a help. Part of being a good follower is the ability to express new ideas and ways of doing things while submitting to the final decisions of leadership. They should also have the integrity to not go poking around in places where they shouldn’t, even if they have the power/permissions to do so (same goes for IT personnel).
None of this addresses liability or legal issues. You probably have systems to which volunteers should not have full access. This may include financial info, Personally Identifiable Info (PII), or whatever. Something to consider.