I used to do road biking, but I imagine the same will apply to mountainbiking. Find a group ride in your area and start going on it consistently. On the calm stretches there's plenty of chance to pull up alongside people and have a conversation. Push conversations beyond mountainbiking, learn about these people and teach them about yourself. When you find someone you like, who seems to also like you, invite them to hang out outside the group and take it from there.
The hard part is getting the first 1-3 friends to bootstrap your social network. Once you're at that point your friends will introduce you to others, and your network will blossom organically.
The hard part is getting the first 1-3 friends to bootstrap your social network. Once you're at that point your friends will introduce you to others, and your network will blossom organically.