changing guild during territory battle

If you join a guild after TB or TW has started you are locked out of the event. Much the same as you are locked out of raids.

Don't guild hop. Find a guild that suits you and stay in that guild.

@GunsAtFour is correct, if you join a guild after the start of territory battles, you will be unable to participate in the TB.

If it is in preview phase you can still join

Territory Battles have no preview phase.

Unfortunately you would be locked out of rewards. Similarly if you leave before it's over, you will be locked out of rewards.

The compressed schedule for the next couple of weeks makes it very hostile to players who are leaving/joining guilds.