I guess both. Sometimes tribes having hard times would join better off tribes. Naturally women would be expected to procreate. Taking women during raids was common too. Or if men were killed in warfare, women may have "voluntarily" joined the other tribe.

This is common to overall tribe relations. For example, Baltic and Fino-Ugric people in North/east Europe. Baltic people came as part of Indo Europeans while Fino-Ugric people lived there before that. Modern Lithuanians and Latvians have much more Fino Ugric genome in women-specific parts. Some of traditionally women arts were also carried from Fino Ugric tradition into Baltic. Conclusions are pretty clear...