I would recommend allowing people to just pick a name and hop in. One of the keys to the success of web games is that there's (often) literally no barrier to entry, including the need to create an account.
A recent web game on HN ("Pounce") did the first part well: https://news.ycombinator.com/item?id=31073332
Also I suggest making it really easy to invite someone else to play with you by sending them a link. That's more complex though.
Also since you already have a spectator mode, it'd be cool if people see an active game when they go to the home page, like lichess.org. That might be even harder to do that invite links.
Nice that you can share the result of a game! http://www.battle-of-flags.com/match/index/695
You could automatically assign a throw-away generic name, and then slap a 'try it right now' type button prominently on the home page. If the game is good, then I'd be willing to create an account.
I opened a tab to give it a try, but backed out when I had to provide an email to see how it played. Assuming I enjoyed it, I'd then be motivated to make an account to keep track of my play.