this is bad design. you should have 3 tables:
members
events
links (links together members and events using primary key fields from the other 2 tables).
you should do a google serach for many-to-many data relationships.
but anyway. if you must do this the old way:
$string = 'bob,mikey,jack';
$count = count(explode(',', $string));
echo $count; // outputs 3