how to assign users from contact listpage to selected records for listpage i will expalin my story clearly below

i want to select records from the list page then click multi assign then open page that have list of contact (user) here select the no of users then assign those users to selected records.
for example if i selected 4 records from list page then click button then its showing page there have contact list by user name there selected 4 users then click assign button then have to assign users to respective records.

in image 1 select the records then click multiple users

in image 2 then opened page there select users then click assign 

in image 3 this is the one record there assignee need to set selected users. so gudie me how to achive this confussed in bp like selected records Id are save in bp collection (nested parameter is Id) while using subprocess only can run those selected id one by one.