Depending on the scheme you have in place, there are two ways that employees can contribute:
Relief at source method
Employee make their contributions after they have been taxed via their PAYE. And then basic rate tax relief is paid into your pension at the pension providers end. Higher rate tax payers can reclaim the additional rate via their self-assessment.
Salary Sacrifice
Employees sacrifice part of their salary to be paid into their pensions before tax. For technical purposes, these become a company contribution because the company makes these contributions direct. Companies do not have to pay National Insurance (NI) on the amount their employees sacrifice. So some elect to pay part, or all of this saving into the employees pension to give the contributions an uplift.
For a more in-depth look at the pros and cons of each type, as well as financial examples, read our article on Salary Sacrifice pension here.