1. We would like to have a custom field which has all users that are part of the project, i.e. project specific user
As As of now we have a user picker list which shows list of all JIRA users (whether they are part of user’s projects or not)
You'll need some coding to do that. There are some plugins available that might help you, but you need to think carefully about what you mean by "all users that are part of the project"
Also, you've said you're using OnDemand, so you can't install plugins or code... (yet)
Not for a custom field - the possible combinations of what you might want in there are far too vast to code for.
If you use the reporter or assignee field though, you have complete control over them, because they have clear rules. The reporter field is limited to "people who can create an issue" and the assignee list is limited to "people who can be assigned an issue, as determined by the projects user lists and permission scheme"
Anything else, you need to code for, because there's no fixed definition for it. "People in the project" could easily vary from site to site.
You can see some examples in the marketplace - the obvious ones are "role based picker" and "group based picker", and if I were Atlassian, I'd be looking to add them to the core of Jira. But other selection rules are pretty much too undefinable to be useful as a general rule.
That's nothing to do with your original question.
The layout of fields on screen is nothing to do with their option content. You'll need to amend code to re-arrange the screen to get another field on the same line as assignee.
If you meant you want a field the same as assignee, but not assignee, then please re-read the conversation.
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