A user with login xx@MyDomain.com was first only using a external Yammer network. Let’s call it XYZInside.com network. Than the company of the user created an own Yammer enterprise network. Let’s call it MyDomain.com. The MyDomain.com, The user is now also joing the Yammer network of its company. The Yammer network with the DNS suffix MyDomain.com became now automatically the primary network for the user because of the domain suffix for his login and of the Yammer network are the same.
The user now was disabled in the external network XYZInside.com and could not be reactivated.
Solution: unchecked the option: “Require admin approval for MyDomain.com members to join other companies' external networks.”
Only available in Yammer enterprise networks
The user could join the external network in the past because of he did that before the his now primary network with this policy enabled was created.