Hide Non Activated Users From Member List

[ZiNgA BuRgA]

No.
Please read up on MySQL indexing.  Full text indexing is something completely different.

It would be something like

PHP Code:

$db->write_query('ALTER TABLE '.TABLE_PREFIX.'users ADD KEY `usergroup`(`usergroup`)');

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EDIT: actually scrap that, MyBB already has an index on the usergroup column, never knew that.
In other words, you don't need to add the index.