set your default registered group so they can not edit their sig under user group permissions
View attachment 40055
then create a new user group that can edit their signature
View attachment 40052
then create a new promotion
View attachment 40053
set the promotion to add a member to another group (I used canSig so that is what group I would select) and set how many messages they must make before being added to the group that can edit their sigs...i selected ten here
View attachment 40054