This can be tricky to troubleshoot, given the number of possibilities for the problem. It is also version dependent. In Jira 6.4 and earlier, you use the "Jira User" Global Permission to grant login access. On Jira 7.0 and later, you do the same thing by granting Application Access. Make sure they have access to log in first. Next, you need to have at minimum, "Browse projects" permission to see and do anything with the Project in question. You will need to look at how the Browse projects permission and other relevant permissions are granted and make sure the user is listed there. To get someone to show up in the Assignee user picker, they need to have both the "Browse projects" and "Assignable user" permission. Hopefully that will point you in the right direction.
There is a simple answer, but
a) Jira doesn't ship with a default set up that allows it to be done easily. You need to change that as a one off exercise
b) When explained in the most simple way, a LOT of people just don't understand it.
But despite B, the answer is "Create a user who has access to that project, and no access to others"
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