Once someone plays their game in a given week for your team, whether on the bench or not, they become "Locked". You'll have to wait until the players become unlocked to sign anyone.
Or, as ESPN.com's hard-to-find but helpful help page says
Once the roster is locked in Standard leagues, you are unable to make any roster changes. League Managers in Custom Leagues are able to make changes to rosters once they have locked.
Individual slots lock at the game time scheduled for the player in that slot for each matchup period. This occurs even if the player is not playing, so make sure you move injured players prior to their team's scheduled game start.
Individual roster slots lock when the team of the player in that slot is scheduled to begin their first game of the matchup period.