and ..... No Answers Yet... If it's impossible, please mention.
Hi Kogul Vimaleswaran
Hopefully someone else can jump in here as it's not something I've done from SharePoint Online External Access. I'm confused as to how far you want to take the access. You want them to be able to update the information you have on them in a list but you don't want to give them as O365 users?
You can have public anonymous users submit information to Office 365 if you have a Nintex Workflow Cloud form front ending the solution and write the information to an Office 365 SharePoint Online list.
If you need to be populating form controls then investigate the Query String, something along the lines of Pre-populating your form controls with Query String might be what you need. But it's having that form visible to an unknown recipient which will be the issue.