I've been playing in a Reggae/Ska band for yonks. If it's Ska then mainly your playing Rock 'n' Roll Bass lines. For Reggae what you don't play is as important as what you do play, leave holes, particularly on the first beat of a bar (though not all the time) and don't be afraid to stop playing for a whole bar or 2 occasionally. Most important if you don't like Reggae or haven't heard much then you won't be able to make a decent fist of it.