If I recall correctly, the Apache license has some kind of clause to the effect that all contributors automatically grant you all the same rights available under the license, meaning that by contributing, they acknowledge the license and give you permission to make use of their contribution. There is also some anti-patent stuff.
http://www.apache.org/licenses/LICENSE-2.0
OTOH, it's a big, long license, and perhaps the BSD license would be simpler, which is always an advantage.