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The easiest way to contact all your users is to use the "Mail Users" feature of Xoops.
a) Go to Admin > Mail Users.
b) Select the group(s) you want to send the message to
c) Enter the Subject and Body text
d) Select the "email" checkbox in the the "Send to" field.
e) Click the "Send" button
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The easiest way to get a "copy" of all your users emails is by using phpMyAdmin (or similar). Do a query on the database and then export the results.
The query would be something like:
SELECT name, email FROM `yourtableprefix_users` ORDER BY name
This query will return the users name and email address. You can then export it to whatever format you need (say CSV) and then go from there.
Note: Replace 'yourtableprefix' in the above query string with your XOOPS table prefix.