Will every believer be part of the army of Imam Mahdi?

Assalamualaikum, I've been thinking about something and wanted to ask. When Imam Mahdi (may Allah hasten his reappearance) returns, will every single one of us believers have to join in the fighting? Like... literally everyone? I mean, even people who aren't really fighters or are physically unable? How's that gonna work exactly? Just curious what others think about this.