meta_query for a string inside a meta field containing a comma-separated list

Is there a smart way to meta_query so it matches just “IS, but not
“TS-IS”?

The WordPress meta query class (WP_Meta_Query), which among others, is used with the posts (WP_Query) and users (WP_User_Query) query classes, supports REGEXP (since WordPress 3.7) as the compare value, so you could use that with a RegEx (regular expresion) pattern like (^|,)IS(,|$), like so:

'meta_query' => array(
    array(
        'key'     => 'usergroup',
        'compare' => 'REGEXP',
        // With dynamic values, be sure to properly escape the "IS" or whatever
        // is the actual value. E.g. '(^|,)' . preg_quote( $value ) . '(,|$)'
        'value'   => '(^|,)IS(,|$)',
    ),
),

So that should work well for meta values that in the database are stored in the form of a comma-separated list, i.e. value, value, value, ... (with or without the whitespaces), and you can test the above pattern here.

However, since you said (in the comments) that the meta values are actually serialized like so:

a:11:{i:0;s:2:"IS";i:1;s:5:"TS-IS";i:2;s:6:"TS-USA";i:3;s:2:"TS";i:4;s:3:"EKG";i:5;s:2:"WS";i:6;s:2:"CT";i:7;s:6:"TS-MAX";i:8;s:5:"TS-XD";i:9;s:6:"TS-CTM";i:10;s:5:"TS-MT";}

Then you would instead want to use LIKE as the compare value, but use "<value>" as the value‘s value like this:

'meta_query' => array(
    array(
        'key'     => 'usergroup',
        'compare' => 'LIKE',
        // With dynamic values, be sure to properly escape the "IS" or whatever
        // is the actual value.
        'value'   => '"IS"',
    ),
),

Try a demo on DB Fiddle.

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