Very quickly, in Baldur's Gate 3, you'll be presented with multiple companions to have in your party, with more unlocked in the early hours. If you chose an origin character, you'll still quickly get a four-person party, but if you created a custom character, you'll have four additional companions to fill out the remaining three spots.
While you can only have four people, including yourself, your extra party members will be waiting for you in your camp; here's how to switch them out with your active party members.
How To Switch Out Party Members
The only way to switch out party members is by returning to your camp and talking with them. By asking them to join your party, they will tell you your party is full and ask you to leave someone else behind. To remove someone from your party, go up to them in camp and ask them to leave your party and stay in camp before asking the new party member to join. This can be done as often as you'd like, and there is no real penalty to switching out party members. It's important to give everyone a fair shot in combat to see which party composition is best for your play style.
The only way to go to your camp is by completing a long rest, or using an Ancient Sigil to travel there. You won't need camping supplies to fully complete the long rest just to talk to your companions, letting you easily switch them out without burning through your supplies. Your decision can also be changed as much as you'd like while in camp, but once you leave, you'll need to complete another long rest to switch them out.
Inactive Party Members
Inactive party members will wait for you in camp and will be there whenever you need them. While inactive, companions in your camp will gain the same amount of XP as the rest of your party. Upon joining your party again, they can level up as many times as they have XP for, making them strong enough to be in your party whenever you want. While they get XP while inactive, they won't be getting new gear, so set aside some new gear or pull other gear off your soon-to-be inactive party member to ensure they are strong enough to join your active party.