WJBender made a navmesh plugin called Peragro of the Recast/Detour/DetourCrowd. By the way, it is available in Unity and UE4 by default. I don't know whether it is still available or not, he switched to Unity. It has good performance I think only max 100 characters. To make your own plugin:
http://masagroup.github.io/recastdetour/
https://github.com/recastnavigation/recastnavigation

another open source solution is RVO2:
http://gamma.cs.unc.edu/RVO2/

by the way if you need larger crowd, you can implement a vector field pathfinding and movement system, or use simply a potential field. I have recently implemented my solution based on this, but it does not deal with avoidance (but possible to add little repulsion to the heat map around units):
https://gamedevelopment.tutsplus.com/tut...g--gamedev-9007

But it really depends on you what you mean by crowd, and what is the exact purpose, what limitations you can accept on quality and performance...