According to this site http://www.worldtaximeter.com/ibiza/san+antonio+bay,+ibiza/playa+d'en+bossa it's not too far

The resorts are on opposite sides of the Island. By car you would be looking at 45 minutes at least. By bus you would have to get a bus from PdB (or walk) into Ibiza Town then get a bus across the island. Then it would depend on how far your friends are staying from the bus stop.

ok by car back road or main highway 20 mins.by bus you get the 14 into ibiza town,around 1.45 euro.then when you get off walk up the street untill you see the main bus staiton office.walk into the left side.last box.get ticket around 1.95. bus number 2.walk out of office turn right.about 25 yards outside a clothes shop is the bus stop.get on and away you go.20 /25 mins depends on traffic.get off at san an bus station.but like said depends if they are staying in the bay.you can get another bus there.but its a nice walk round.
follow the people down a alley that leads to the egg island.you could meet up there.
you pay to get on the bus in pdb every 20 mins.you must get a ticket in ibizatown to get on the san an bus.same coming back walk into the office and look for bus 2 desk.

the number 2 runs on the hour and half hour.but more are put on at peak times.so in all you can be in pdb and get to san an in around an hour.taxi would be 27 euro one way.

when you do come back and get off bus in ibizatown.walk down back to where the 14 bus dropped you off to get bus back.there are numbers on the bustop to help you.you may and may have to pay at kiosk or the driver.
you can walk it around 25 mins back to pdb.more if you right into pdb.walking through figueretas on to the prom and then back street then through a allley way that leads onto the beach at pdb.