Rosters aren't locked. They tend to be locked at about the 10th round of the season. As roster movements on Sundays are prohibited, Caer locks out the powers of captains/players to adjust the rosters themselves. In stead all roster adjustments just go through Caer or another mod with bot access. In short: you can still join and leave rosters until the 10th round (we're currently in Week 2), just not on Sundays (gameday).