Yes, you can invite other guests, but here is the key: to be eligible to reclaim VAT and for corporation tax relief, the costs must solely be to entertain employees.
Former and previous employees don’t qualify, nor do subcontractors and nor do shareholders who don’t work in the business. An employee has to be someone who is on your business’s payroll and being paid a salary so this would also exclude clients or friends of employees. If you want to entertain a mix of employees and non-employees at the same event you can, however you will only be able to claim tax relief on the costs of the employees’ entertainment.