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I managed this with simple SQL inserts, generated with an
Excel-document. In column A i put the "Full Name", the
e-mail for the user in col. B. In the C-column, I put a user
ID, increasing by 1, starting with the current largest user
ID (you get it from the table user_sequence).

That is:
A2: Nathan Newuser
B2: nathan@user.com
C2: 31 (formula: =C1+1)

Then you need to know which sid your survey has; in my
example 29. For normal users (just premitted to answer, the
status will be 1, and the results_priv = 0).

Use Excel to generate SQL like this:

INSERT INTO users (uid, sid, name, email, status,
results_priv) VALUES (31, 29, 'Nathan Newuser',
'nathan@user.com', 1, 0);

You can do this by inserting the followin formula into col. D2:
="INSERT INTO users (uid, sid, name, email, status,
results_priv) VALUES (" & C2 & ", 29, '" & A2 & "', '" & B2
& "', 1, 0);"

Then you copy the ID-column (C) and the SQL-generating
column (D) down as many rows as needed. Mark all data in
col. D, copy and paste into your SQL manager/mysql-prompt.

Then; at last update the sequence-table using:
UPDATE users_sequence SET id=xx
XX = the highest value from your col. C (user ID).

Works very fine for me, hope it will work for you too :)
Best regards,
-Bakkulf