Oh okay so when they're still on waivers they can be signed to someone elses ps.
Up to four players can be protected on the league's practice squad each week; Other teams may not sign protected practice squad players.
Players who are on a team's practice squad may not sign a contract with another team's practice squad. Instead, the signing team must sign the player to a 53-man roster spot. Those players can sign with another team at any point during the season.
I believe a PS player, when he's actually on a PS, can turn down an offer from another team. I've heard it happening before. If they are signed from a PS, though, they have to be placed on the 53 immediately.