Well this is kind of a tutorial, or a list almost of what people without the modded map can see. Meaning if the host has modded thier map and other people didn't download it. It's going to be almost like a project I guess since I'm going to try and add as many helpful things as I can. Yes I know the tutorial is long, but you won't regret reading it. (At least I hope)
Techniques(for lack of better word):
PMI:As far as I know, they are not joiner friendly, or at least they will not see ANYTHING you PMI'd.
SP>MP or MP>SP:This I know is 100% joiner UN-friendly.
Lincoln Logs:Joiner friendly to an extent... According to my friends ingame, they could not see the logs, and I noticed when they were in a log box they couldn't see me when I was right infront of them.
Tools:
HMT: If you stick to mainly basic things such as changing a projectiles force, it should be joiner friendly, as long as you try to avaoid meta swapping and don't PMI.
HHT: I have had no problems with this so im going to give this a 99.99% joiner friendly.
Eschaton: Never really experimented too hard with this, but same as HMT, I think as long as you don't get too advanced with the mods it should stay joiner friendly.
Sparkedit: Scenery is not seen, meaning if you move a tree they will still see the tree in its usual spot and when they hit the new tree they lag like they keep moving but lag backwards to were the tree is. Teleporters are 100% joiner friendly. And vehicles and equipment and MP flags and spawn points are 100% OK to be edited and are joiner friendly.
Things To Avoid:
Changing the speed of vehicles or the speed, jump height, or walk on walls ability of the Biped itself, they have a very uncool effect in-game for joiners. Remember those people who host DevTrainer servers? Remember that annoying lag with the super jump? Its like that lag, but moving on the ground... the people who don't have map won't get an exception error, it's just not a fun modding experience for them.
Edited ROF's usually go unseen. Take the the [BC]Dangercane map for example, the pistol is really shooting at high ROF, but if you don't have the map the pistol is shooting at normal speed, you just hear the DING of damage being done.
Projectile Speed changes, they will still see the projectile moving at standard speed, except the object they were shooting is damaged faster and the projectile dissapears once it really dies.
Model swaps will also go unseen, no matter if you import a new model in HMT or just swap the Mod2 in HHT. They will not see it.
Bitmaps or decals or any effect change and so forth, such as flaming trails on the hogs, or maybe a flame grenade, or maybe you made the warthog with your friends name on it. They will not see this.
Fun Things To Try:
These next bunch of notes or whatever you can call them are just a bunch of different mods that I have hosted before that alot of people really loved.
Flying every vehicle, by setting all vehicles except for the banshee to fly like a pelican. Many people enjoy this mod, the only thing for almost every new person that joins, you must explain how to control the vehicles well or else they fly really fast into walls and complain. Also you must turn on the Immune To Falls checkbox in the BIPD tag. If you dont know what the Pelicans controls are,
Space=Vertical Upwards thrust
Control=Vertical Downwards Fall (landing)
W=Thrust Forward
S=Stop
Just a little note, for most vehicles (especially the tank) you need to do a complete 180 degree turn and be looking forward while landing to land rapidly while going nearly full speed.
Teleporters, this requires a little skill in my opinion. You need to not over place the teleporters or else it will take people an hour just to know half of the teleports. You also need to place them in good positions, so you don't have to run an obstacle course just to teleport to the middle of the map. Also make the teleporters fair, if its a map that has a mostly team balance, try to balance out the teleporters so that neither team has an advantage. If its a map meant for FFA you can probably just place them anywhere though.
Changing the force of weapons, alot of people really seem to like that. Where every weapon has a force to it, and I'm not talking about making the snipers shoot tank shots

Well I'm going to keep adding to this as I think of new ideas or tips for you guys, but until then hope you like this tutorial and keep checking up on it for new additions. OH and please vote if you liked this or not

Thanks
-Swampy117