Active TF2 Mix / PUG Groups

The main competitive formats are 6v6 (classlimit 1 except for scout, soldier) and 9v9 (1 of each class).
In competitive formats you're generally expected to use a mic to rely info to your teammates, and also listen to their calls. You are also expected to cooperate with your team and be more objective-oriented than in a pub match.

6v6 groups:
Beginner-friendly 6v6 PuG sites:
Global: tf2center.com - Low quality pugs of all gamemodes, no mic required
NA: TF2 Coaching Central - https://discord.gg/XZtCce4w
EU: Eu TF2 Newbie Pugs - https://discord.gg/GVnfBfbffQ
SA: MIX TF2 BR NOVATOS - https://discord.gg/sbnwVG9cMh
NA/EU: TF2C PuGs - https://discord.com/Z7QRYGt8Yn

Intermediate 6v6 PuG sites:
PL: https://tf2pickup.pl/
FN: https://tf2pickup.fi/
CZ: https://tf2pickup.cz/
DE: https://tf2pickup.de/
RU: https://tf2pickup.ru/
BR: https://br.tf2pickup.org/

Advanced 6v6 players only:
EU: https://propugs.com/main
NA: https://pugs.tf.plus/lobby

Highlander groups:
Global: tf2center.com - Low quality pugs of all gamemodes, no mic required
NA: https://pugs.tf.plus/lobby/HL%20Open
BR: https://hl.br.tf2pickup.org/games

Unverified groups:
Central America - Mixes Slash
Asia - AsiaFortress' discord

No restrictions 6v6:
The only way to play without competitive rulesets and restrictions is to play on the subpar Valve 6v6 competitive MM you see in TF2's main menu. You will find cheaters and have a worse experience than in any of the sites listed here. RGL.gg ran a NR 6s league but it quickly lost popularity and was never taken seriously to begin with.


Addendum: On your first few comp matches, its important to let people know that you're starting from scratch and haven't played competitively before. This will make them more understanding and they will be happy to teach you if you show interest in learning the format and your role.
Ideally you could ask somebody to be your mentor and have him explain the basics of comp and of the role you want to play, or send you useful beginner resources you can view on your own.

Gaming time: Keep in mind that most comp PuGs and scrims start around 7-8 PM in each region's respective timezone, at least for Europe and North America.