Volunteers normally carry out unpaid work for organisations. A volunteer should not be paid for the services they provide, to avoid creating an employment relationship (ie a formal relationship between an employee and their employer that grants the employee various rights, for example, to be paid the National Minimum Wage).
Volunteers can, however, be reimbursed for any legitimate and reasonable expenses (eg for food, drink, travel, and equipment required for volunteering). Take care to have clear expense rules for volunteers. If volunteers receive any other payment, reward, or benefit (eg promises of paid work in future) as a result of their volunteering, this may result in them being classed as workers or employees. Consider creating an Expense policy, tailored to account for volunteers, to set out which expenses can be recovered and how.
For more information, read Working as a volunteer.