As the law currently stands, European Economic Area (EEA) nationals and their family members must have already been living inside the UK before 31 December 2020 to the European Union Settlement Scheme, though family members are expected to be able to join them after this date. Unless the current deadline is extended, EEA nationals newly arriving in the UK from 1 January 2021 will not be able to apply to the EU Settlement Scheme.
It is expected that only close family members (see here) and not ‘extended family members’ (see here) will be able to arrive in the UK after 31 December 2020 to join already resident EEA nationals. At the time of writing, there was no clear deadline by for family members to arrive by. Get up-to-date advice if this could affect you.