How to setup two mysql servers with group replication to work individually, when no network
Group replication is not a good match for scenarios where server diverge in time.

What you want is a Master to Master setup, where given two server M1 and M2, when the network is restored, M1 is a replication slave of M2 and M2 is a replication slave of M1.

A crucial details is that both servers must not do conflicting updates to the same data, so you should partition your data accordingly and you can also use auto increment settings to generate different ids on different machines.