You can pick some chords and play softly - but it will sound not good when you change chord and the others don't. Follow the chart that's being played, and change chords when the others change. You can keep them simple - triads will suffice, but be careful to use minors and majors when appropriate. Also be aware thhat when there's a ♭5 chord, you shoulddn't be playing the normal 5 for that - it's flat.
Eventually, if you want to add in some extra notes, as in changed chords with extensions, or slash notes, they can go into the mix.
But initially, ask the lead pianist what he prefers under what he plays - and that might vary a lot number to number. Always listen to what's going on, and be prepared to play something that adds to the song rather than fills it too much.