Grandparents wants me to volunteer to sign joint custody

i am a good dad i have never refused the grandparents time w/ my child after the mother & split up . when i found out that the mother was not being in childs life i did not keep them from my child the grandparents & i never had a problem until i refused to sign their papers from their lawyer on joint custody now they have enrolled my child her name is[ Layla] in preschool. i am not understanding the answer about them playing a bait & switch ploy i am a father trying to learn my rights & learn as much from the internet can you explain they are playing a bait & switch ploy? i am glad for all the advice every one has given me