Hi
Group VPN limitations still apply, so unless all your networks behind SRXs have
public IPs, you can safely forget about group VPN.
To do full mesh, simplest setup will be to just create tunnels from every to
every SRX. So 6 SRXs, 5 tunnels on each, 5*6=30 tunnels. You do not need
a tunnel for every network behind SRX if you are doing route-based VPN
(recommended approach). You will need to create a unit on secure tunnel
(st0) interface for every VPN tunnel, for example st0.0, st0.1, .. , st0.5 - on
every SRX, and bind the configured VPNs to these interfaces. Don't
forget routes (static or you can use OSPF for example). Also don't
forget to put st0.x into zone and write an appropriate security policy.
You can do hub-and spoke topology as well, if you like. Then there
will be less tunnels and all traffic will go through one site (hub).