No. To be eligible to participate in war, you must be in the alliance by the time the officers select "Enlisted". This is what records who is in the alliance, even before matchmaking starts.
If you've selected "Enlisted" and then have a new person join, you can cancel the enlisting and re-enlist, if prior to matchmaking starting. If you fail to do this cancel and reenlist, even if they joined prior to matchmaking, they will not be able to join. Once matchmaking has started, however, nothing will allow a new member to fight in that war.
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If you've selected "Enlisted" and then have a new person join, you can cancel the enlisting and re-enlist, if prior to matchmaking starting. If you fail to do this cancel and reenlist, even if they joined prior to matchmaking, they will not be able to join. Once matchmaking has started, however, nothing will allow a new member to fight in that war.