MegaTF players come together in many ways. Often, several players who are extremely good at playing a particular class come together to share techniques and insights into how to better play the class. Take for example, scouts. Many clans have players who are very proficient as a scout. When a group of these players get together and form a guild, they can learn from each other, become better players, and recognise each other in the community.
Players in guilds will often attach letters to the end on their name. 'VI' for example, is the tag for Velocity Intake, an elite scout guild. Members of Velocity Intake would list their names as follows:
Member Name Here.VI
Many guilds are invite only. This means that they will not accept applications, and will only offer other players membership if the guild leaders see them display great talent. Other guilds require you to submit a demo of your play to the guild leader for review.
When you see players with letters and symbols following their name, it often means they are in a guild.
There are two active MegaTF leagues at the moment. One is Team Pro League, or the TPL, in which many clans are currently participating. Joining one of these clans may give you an opportunity to play in a TPL match.
A 3 vs 3 league has started recently as well, named Justice For All. Anyone can enter, just get a few friends on your team and fire away.