Though many leagues have their own rules, so there are some differences, they all have to adapt to the fact that some of their incoming players may be off playing in the NCAA tourney.
So, they will routinely offer 10 day contracts to players who aren't signed in order to survive until everyone arrives. It's very common at the beginning of the summer. Don't be surprised if the roster "explodes" for a week or two. As the players filter in from college ball, the ones under the 10-day contracts will leave.