​@Vsevolod Tsurikov  If the conversations are unassigned or assigned to.a team, this setting in Assignments should help:

Self Assign will assign the conversation to the user if they reply to an unassigned conversation, Is that what you were looking for?  Or do you need an option to auto reassign a conversation to another team member if they jump in to reply on a thread that was already being handled by another agent?

​@Nathan Sudds Thank you for the link. In my scenario, the conversation already assigned to another user. Ideally I should be able to orchestrate if a replying user should be added to the list of assignees OR replace and existing assignee → second scenario you described.