Group pickups are done from available allocations. This means that extending a reservation will answer the same rules and if there are not enough allocations available, the extension will be denied. This setting enables the possibility to extend a reservation or even add a reservation to a group even if there are no allocations available. This setting will add any missing allocation automatically to the group.

Path: Management > Setup > Property > Parameters > Section "Parameters" (right side)

Use the drop-down to set "Res to Alter Group Allocation" to Yes.