Before adding users, you have to specify allowed domains in the dedicated section.
You have 3 options to add users :
Manually by writing emails one by one and pressing enter between each email
Via CSV, by importing a CSV file with a column containing the email list. The file must contain only a column with the email as a name
Via a registration link, that can be shared and contains a token allowing any people with the link and an email of the authorized domains to create an account.