Multi-player company membership was added in the 0.7.5 update, titled "Invite other player to Company, New Logging Area with 6×6 Truck!".
What members can do
Invited members can drive company vehicles. They share the company's vehicle pool while the owner retains ownership.
Server toggle
On dedicated servers, admins can additionally toggle whether a player may join the server while driving a company vehicle.
Related
Founding a Company — minimum requirements.
NPC-Staffed Fleets — passive-income workflow.
Corporations — dedicated-server upgrade.