Hello and welcome to the community.
A group flight is exactly what it says. Most members in the community often develop friendships with other users around the world. Here, through private messaging you are able to coordinate a flight with a friend or several friends on one of the servers. The more involved you get, you may find yourself in the official IF discord, if you’re a pro subscriber. This is a great way to connect with new people. Everybody here shares the same passion, so your chances of flying in a group are far higher.
Another alternative to this, is looking into joining a VA (virtual airline). Most VAs organize their own group flights through internal events. You can look at those in the VA category.
Dan, one of the staff members will also create threads here notifying of a community group flight. Another avenue of joining in.
Since you’re also new to the forum/community, I’d like to drop a link that is a guide here Guide to the Community. Check it out, and get yourself familiar, and join in on the fun, conversations, voting, and so much more there is to offer.
Have a great day!