Any non HRE member that borders a HRE nation and is christian has a chance to be asked to join if their relations with the emp are 100 or more.
Any non elector member of the HRE can be made an elector if there's a space, a higher chance is given to those with better relations with the emp.
Any non elector member of the HRE can be made an elector if there's a space, a higher chance is given to those with better relations with the emp.