The fastest way: one link
Most attempts to play chess with a friend online die in the arrangements. Make an account here, add me as a friend, tell me the code, no the other code, is it your move. By the time it works the evening has moved on.
A challenge link removes all of it. You create one, you paste it into whatever chat you are already having, and the first person to open it is your opponent. You do not pick a time control up front: the speed is agreed between the two of you once you are looking at each other, which is quicker than it sounds and settles arguments before they start.
When your friend is not around right now
A challenge aimed at a specific person behaves differently from an open link, because the situation is different. If they are online they get it immediately. If they are not, it waits up to twelve hours and reaches them by push notification or email, so a challenge sent at midnight is still there over breakfast.
Nothing is lost if it goes unanswered: challenges expire on their own, an unanswered one costs neither player anything, and you can withdraw one whenever you like.
More than two of you
For a group, a lobby is the better shape. Everyone joins one place, and pairings happen inside it rather than across the whole site, so you only ever play the people you came with. A private lobby is reachable only by its link, which is what you want for a work team or a group of friends.
A lobby can also run as a timed arena: you set the length, people play as many games as they can fit, and it ends with a standings table. Finished tournaments keep their results, so there is something to point at afterwards.
Ratings, rematches and keeping score
A game from a challenge link is casual: it does not move either player's Glicko-2 rating. That is deliberate. A game against a friend should be able to be a bad game. If you want a rated one, both of you can go through matchmaking instead, or a lobby can be created rated.
Rematching takes one tap, and ChessChat keeps the head-to-head record between the two of you, so the running score exists whether or not the rating does.