Depends on the point buy you used (if any) or the stats your players rolled. If they rolled for stats, use this point buy claculator to find out what point buy equivalent they are.) CR assumes a party of 4 with 15 point buy. So if you have one additional player and probably a higher point buy (since most people seem to do that), you should adjust the APL by +1 and treat them as if the APL was 6.
A challenging CR is APL+1, so your CR should be either 6 if you have a low point buy or 7 if it's high.
Also remember to use multiple enemys most of the time, since with 5 full round actions (+ any eventual familliars, mounts and companions)
the party would probably overwhelm a single opponent. Or said single opponent would surprise-kill one or two players with good rolls.