Here is how to add members to GNU Mailman's lists with a plain text file.

* "Mailman" in this post means not Mailman 3 but Mailman 2, because I use OpenBSD.

The add_members command is the way.

Create a file to write members of a list in it. Its name is, for example, mailman-xxx.list:

user1@some.fqdn
user2@some.fqdn
user3@another.fqdn
...

Run add_members for $LIST_NAME (replace this with your list's name):

# /usr/local/lib/mailman/bin/add_members \
    -r mailman-xxx.list $LIST_NAME \
    # disable to send welcome messages to new members \
    -w n

Then, your members are printed as registration success:

Subscribed: user1@some.fqdn
Subscribed: user2@some.fqdn
Subscribed: user3@another.fqdn
...