Due to personal experience which contradicts the current "Meta" of Maplestory,
I created a handy little spreadsheet to help players decided when it is and isn't viable to grind with a party.

You can find the spreadsheet >>>here<<<
It is important to note you will have to make your own local copy to use this calculator.

Scroll Down for Images and Examples.

A few things to note about the calculator.
1. The calculator assumes that no one other than you attacks your own mobs.
In practice this will most likely not be true, but it's not that important as the damage you deal only accounts for 20% of the base EXP adjustment.
The other 80% is based on your level vs the party level.
2. The EXP Bonus Field is in %, rather than multiplier.
This means that a 2x coupon by itself would be written as "100" rather than 2.0,
I feel like when adding things like Gold Pots, Holy Symbol, etc, this is easier to use.
3. I have no clue if this will work outside of google sheets or not.
I don't think I've used anything native to google, but a fair word of warning is, I haven't tested it.
4. The Calculator does not stop you from creating an impossible party.
You'll have to make sure no two characters are more than 40 levels apart.
You can look at the box near the top that says "Party is Okay" to make sure you haven't done this.
It won't disregard inactive members though, so be aware of that going in.
5. The Calculator does not factor in reboot bonus base EXP.
Reboot base EXP is calculated differently than regular servers.
Please use the correct values when calculating for reboot worlds!
6. This Calculator does not tell you how much EXP you will actually earn.
There are extra variables that are impossible to simulate without actually going out there and working with your party.
Things such as Elite Mobs, Rune Spawns and Bounty Portals. Ultimately you'll need to do field testing before you'll know for sure.
However I hope more players will be able to see now that grinding in a party doesn't have to be the hell hole that the meta thinks it is.

Enter the Level and EXP Value of your monster here.


Next, you will need to enter in some basic information about the player.
Their level, Number of Mobs Killed when Solo vs When in a Party and if the Member is a cleric or not.
For ease of use, you can also check the last box to include or exclude that member from calculations.


You can see in this example, a party with one cleric and one non cleric, it is worth it to be in the party.


Here's a more extreme example.
In this example you can see that two of the players lose out on EXP.
A certain balance must be kept in order to make sure EXP is still worth it for you.
In this example only an extra 600 mobs are killed. Which is only 30% extra. I believe this example to be reasonable.
This last example is what you will see when opening the spreadsheet for yourself.


I checked multiple times to make sure every calculation was correct, but in case you notice something out of place, please let me know!

Edit 2018-10-13: Fixed a calculation error where bonus exp was applying exp bonuses incorrectly. Added some more clarification to note #6.
Edit 2018-10-15: Fixed a similar calculation error on "Party Bonus 1", Moved 100 kills from Party Member 4 to Party Member 2. (Due to new EXP calculations.)
Edit 2019-4-22: Fixed a calculation error where the final +1 was not being applied to party leech bonus line, resulting in 1 missing EXP per every party kill.