If you're in the Defined Benefit (DB) section...
The Plan isn't set up for regular contributions from you. But you can top up benefits by Paying Additional Voluntary Contributions (AVCs), which will be invested.
Any AVCs you pay will be invested to help it grow, and this money will be used to provide a lump sum.
If you're in the Defined Contribution (DC) section...
The Plan is a regular contribution Plan and you can pay 'saver contributions' through salary sacrifice, rather than directly out of your salary.
This saves you and Citi money, and because income tax and National Insurance Contributions are based on earnings. By agreeing to lower your salary by the amount being paid into the Citi Plan, both you and Citi reduce what you have to pay.
More detailed information about how salary sacrifice works can be found on MyCitiPension