As with any Mass Effect game, in Andromeda the player can assign up to two companions to his party - they will accompany him during his adventures. Party selection is one of the most important matters, especially because Andromeda introduced a new gameplay mechanic called Combos (more on that in a separate chapter). The game itself won't inform you how you can change the party composition. During the beginning of the game you won't be able to customize anything, as only Cora and Liam will be in your party, but a new companion, Vetra, will join you during the first boarding of Tempest.
To change the composition of your party you have to enter the loadout selection window (it appears each time you leave the Tempest ship, it can also accessed from the Tempest and by Forward Stations on the surface of the planets) and go to the last tab. This shows the current active party members - those that will accompany you during the next mission. To change it, select the character you want to replace and choose the one you want to get into the party. Afterwards, you just have to leave the loadout selection menu, the changes will be saved and your current party will be updated.
Party selection is not only about the combat capabilities of your team - sometimes you will need to take a specific party member on a mission to, for instance, push the quest forward. Specific party members are also required during their loyalty missions.