We play Monster Hunter World

Information taken from this reddit post

First things first, after you complete your first tutorial mission and post your first hunt, your friends can join you (and easily!) on almost any hunt from there on out. However, a few conditions must be met first:

1.) You and your friends must be in the same online session. This can be done in a number of ways; by sharing your session's online code found in the session player list (this changes each time you join a session!), by joining their session via the friends or recently played player list, or by joining a squad session you are both a part of.

2.) The hunters who are trying to join must have completed the requirements to be able to play the mission themselves.

3.) The player who posted the hunt must watch all available cut scenes for that hunt. That is the part that trips a lot of people up. Usually, this is just the introduction for the monster, but there may be one or two that show up later in the hunt. Edit: In addition, the player joining must have also viewed the cutscenes previously.

Once those 3 conditions are met, your friends will be able to join your hunt. To do so they just need to go to Join Game and then Available Quests. Do not use SOS flares for this. Available Quests will show all joinable hunts in your online session only, whereas SOS flares will show a select list of hunts pulled from any online session anywhere. SOS flares are great for if you want to join in on a hunt with random people, or if you need help and will take random people to help you out.

A side note is that expeditions may or may not show up in available games. They are a pain to join, but you'll find they are not worthwhile to do anyway. You get bonus rewards for doing hunts / investigations, so it's better to load up a hunt in the area you need than to go out on an expedition. You can do one together, however, by creating a hunt and finishing it (or failing it) and then both selecting Return to Camp together.