@mywelcome to the community.
Jira's philosophy is that you assign one task to one person, and try to break task so that it only have one assignee.
But in some rare cases this is not possible, for this Atlassian have this guide - https://confluence.atlassian.com/jira/how-do-i-assign-issues-to-multiple-users-207489749.html
Please go through this.
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