Teams allow you to see other members of your team on the map, share markers and chat within your team in the text chat.
At the bottom left of your inventory screen is a button saying "create team". If you click it - a team is created!
Stand near someone and the prompt "invite to team" will appear. Press e and they will be able to click "join team" in their inventory.
Team chat is in the same place as the global text chat. Press tab to switch between global and team chat.