How to build a big and great Lichess team?
There are a lot of ways to promote your team!Name and Description
It can be hard to come up with a good name fast but it should be some kinda original (relatively to other lichess teams). I don't advise overloading the description with photos or gifs, maybe one team logo. Provide general information about the team, like rules, community links (see heading 3), achievements.
Participate in team-battles
If leader of the team joined it in any team-battle, this tournament appears on team page. Firstly, I recommend you to participate in Lichess Liga and Lichess Mega team-battles. Secondly, you can join various official weekly variant team-battles (see them in official variant teams, for example Lichess Antichess). Besides, there are a lot of other organizers on lichess - Ensemo, FischerRandomChess, RoyalChallengersMK, thematic-maniacs, MahendraBK, syams-OPG, emadaly, LAEKU, choose what you like, usually you can just ask to add your team with link to it. Also, you can organize team-battles yourself :) Personally, I have noticed that team can grow pretty fast if it joins as many team-battles as possible, so I recommend to join a lot of them but concentrating (inviting players by sending team-message, playing yourself etc.) on few for example on Lichess Liga and Lichess Mega Team-battles.
Develop the community
Lichess helps you to contact members of your team, you can provide a description, send team-messages and post on forum of your team, but you are not limited to it. You can use other platforms, there are a lot of them on the internet. I recommend you to make Discord server or Matrix room, so people could chat with each other in a convienient way, in addition you can create a team page on any social media that you like (or even create team website if you can) and post news/announsements there. Also, if you like different board games you can expand your team spaces by creating the same one on other free open source websites, similar to Lichess like Lidraughts, Lishogi, PlayStrategy. Don't forget to talk with people that joined your team, make friends, in fact, this is the main thing that the team can give to you :)
Prize tournaments
Should've added it, because if you are willing to spend money(сonsider that pirated e-books and software are prohibited) on the team, you can make it big very quikly. Although, I think that it's better to combine this with previous ways, since it's possible that people can start to join not for your team, but only for your money :D. In general, prize tournaments are always fun, so organize them if you have such desire and possibility!
Be careful with giving a leader!
It's actually a big problem, I've seen a lot of teams crushed (all members are kicked/team closed) just because someone gave a team leader to dishonest person. Manage what certain leader can do, and what can't. It can be much easier and more fun to manage team with other people, but you shouldn't give a leader to anyone you barely even know, only trustful people should be around you and your team.
Spam people so they would join your team!
Just don't do it, I receive like 2-4 such messages everyday (please join my team etc.) and it's just - bruh :/. Not only it's against Lichess ToS so you can get chatban easily, it's just disgraceful. You can invite your friends, acquaintances etc., but you probably shouldn't mass-spam to random people on the internet. You can put link to your team in your lichess profile, I think it's fair enough :)