I have been playing CO-OP with 1 other teammate, and my process is every few hours (as frequently as you can) we save VIA shelter, my friend uses the "Save and Quit" button, and then I quit the game and load back in, then I invite him and we continue. We do this every few hours because we can only do it when we are back at base - I mean we COULD make it away from our base, but we have been encountering a consistent spawn bug. This will have a slight spoiler if you are playing storymode -
There is a drink that you will have to drink to restore your memory, it is in certain locations around the map, I've had it drink it twice so far. The first time, it was 10 feet away from my base, and now that I am on Map 2 (there are 3 maps) it is a bit farther away from my base, but still close enough for me to walk. It seems to be that if you are just starting and haven't drank any of this, your CO-OP Partners will spawn at the starting location when loading in (they will have their gear) and if you drank it, they will spawn wherever they drank it last. I'm pretty sure this is what's happening - When my friend and I play tonight (around 5PM CST, it's 9:39AM right now) I'll report back, he should spawn at the second drink location. When the Host drinks it, it auto-drinks for the other person, so i want to test out if he stands in the camp, and I drink it, will it make him drink it and then spawn him in the camp each time?
I'll report back with my findings, because the spawn bug is SUPER irritating for those who want to save a lot, and even if I can get him to spawn near base it doesn't allow us to save and quit when we are out, unless we want him to go back to base for some reason. You can still make a shelter and save, but like I said I like to relog to make sure we don't lose progress, and I ensure you to do the same. We use the exact same method every time, and we just hit like day 35 or something, and things have went extremely smoothly so far. The only real problems we are having is the spawning, and it's about a 50/50 chance when I throw my spear at a tribal member that is just up and disappears, but arrows don't seem to do that, so I'd recommend making arrows.
I also recommend taking a look at this page - For the first bit, Rain will be your friend, put coconut shells, turtle shells, and whatever other objects you find that look like they can hold water on the ground to catch rain. Eventually there are water filters and collectors, but IMO they aren't very good. We went straight for the mud water collector, and let me tell ya, that thing is amazing, I highly recommend getting one before day 25.
https://greenhell.gamepedia.com/Environment
I was told it was 20 to 30 days before dry season starts. It was 33 days for us, so it slightly varies, but eventually, there will be no more rain, until the next season, (around 30ish days) so make sure you are nearby a water source and if you have the mud collector you can use it to filter 300 units of water at a time, and store 300 "dirty" units of water up top so as you drink it, it fills back up.