Hi @Sakis Bal
Welcome to the community!
Please have a look at the below Atlassian KB and previous discussions.
https://community.atlassian.com/t5/Jira-questions/Assign-an-issue-to-Multiple-users/qaq-p/682480
https://community.atlassian.com/t5/Jira-questions/Assign-a-task-to-multiple-assignees/qaq-p/397690
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