sorry for the wait but...... first; back when the plants attacked a few of the tribe hid away in the bunker and even fewer survived staying behind to fight for their home. once the plants died out the two groups met up by chance, their numbers dwindling below twenty they started to rebuild and repopulate their tribe, as did other tribes scattered right across the planet.
secondly; the tribe refers to anyone not in the 'family' as outsiders, so these invaders are general travelers, other tribes, soldiers and anyone else who enters their land.