Sounds like you are over-pitched to me. When they re-propped your boat did they put the same pitch and diameter prop on? Sounds like they went up a pitch or two, which would probably be OK with you by yourself, but when you add 3 people, the motor isn't able to get into the power band to plane the boat. If you have ever driven a vehicle with a manual transmission, it would be like trying to pull away from a stop sign starting in 2nd or 3rd gear.
I don't have specifics on your boat, motor, lower unit gear ratio, etc. which all play into the equation but based on the information you provided, it sounds like you need to drop one pitch, possibly two in order to plane your boat. Depending on the HP, you may not be able to plane the boat with 3-4 people in it? If you drop too much pitch, you risk over-revving the motor which isn't good either.
Also, prop material makes a huge difference as well. I run a 13x19 stainless on my 70 Evinrude, and have 13x19 and 13.25x17 aluminum props as spares. The 13x19 stainless planes the boat better and runs about 3 mph faster at wide open throttle than either of the other two due to reduced blade flex. This is another possibility as well. Did they replace a stainless prop with an aluminum one, thus reducing performance?
There are several scenarios that could cause what you are experiencing. To better help you, the make and model of your boat, the make and HP of your outboard and the material and size of your old prop and current prop would be very useful information. Not trying to be disrespectful, but without more information all we can do is guess for each possible scenario.
Also, most marinas will have props you can test on your boat before buying. See if you can find a place that does this. Then get a couple buddies to go for a cruise and try to find the correct prop.
Edit: Just noticed you stated it was a 60 HP Mercury..but former and current prop information and boat info would help. I'm not up on the latest Mercury motors, but either way it sounds like you are overpitched or running aluminum when you used to be running stainless.