Another tool to consider that is low cost is Mighty Networks. I’m not sure if this would work as a tool hosted by a company for a CoP that includes employees and external members, but it is very simple and has a great mobile app.
The Chicago Online Community Professionals (a CoP) use it and I am one of the admins / community managers. I am also in other networks / groups using Mighty Networks and I can easily toggle between my groups in the app and online.
