The UK officially left the EU on 31 January 2020. We are now in a 'transition period' until 31 December. During this transition period there will be no change to any regulations regarding employment, travel or residency for EU citizens. So you can continue to employ staff from EU countries as normal during 2020.
However, new rules are expected to apply from 1 January 2021 (this is the earliest possible date: it could be later depending on ongoing negotiations with the EU). So if you currently employ any staff from EU countries you should encourage them to apply to the EU Settlement Scheme for settled or pre-settled status if they haven't already done so. This will enable them to continue to work in the UK from next year. Some EU citizens have experienced unexpected problems and delays when applying for settled status, so people should not leave this to the last minute.
Be aware that many EU citizens are feeling unwelcome due to the current political climate, and may be considering returning to their home countries. We must all do our best to ensure that our European friends and colleagues are made to feel welcome and valued in our communities.
As it becomes available, new guidance and information will be published by the government on its transition pages on the .GOV.UK website. Currently the information is pretty sparse, but one hopes that more concrete details will become available over the coming 11 months.