diff --git a/upup/pkg/fi/cloudup/openstacktasks/instance.go b/upup/pkg/fi/cloudup/openstacktasks/instance.go
index b30d8bb348..edf39c1368 100644
--- a/upup/pkg/fi/cloudup/openstacktasks/instance.go
+++ b/upup/pkg/fi/cloudup/openstacktasks/instance.go
@@ -297,7 +297,7 @@ func (_ *Instance) RenderOpenstack(t *openstack.OpenstackAPITarget, a, e, change
 			return fmt.Errorf("Error creating instance: %v", err)
 		}
 		e.ID = fi.String(v.ID)
-		e.ServerGroup.Members = append(e.ServerGroup.Members, fi.StringValue(e.ID))
+		e.ServerGroup.AddNewMember(fi.StringValue(e.ID))
 
 		if e.FloatingIP != nil {
 			err = associateFloatingIP(t, e)
diff --git a/upup/pkg/fi/cloudup/openstacktasks/poolassociation.go b/upup/pkg/fi/cloudup/openstacktasks/poolassociation.go
index 381a2ace0b..b2f7f91e71 100644
--- a/upup/pkg/fi/cloudup/openstacktasks/poolassociation.go
+++ b/upup/pkg/fi/cloudup/openstacktasks/poolassociation.go
@@ -160,7 +160,7 @@ func GetServerFixedIP(client *gophercloud.ServiceClient, serverID string, interf
 func (_ *PoolAssociation) RenderOpenstack(t *openstack.OpenstackAPITarget, a, e, changes *PoolAssociation) error {
 	if a == nil {
 
-		for _, serverID := range e.ServerGroup.Members {
+		for _, serverID := range e.ServerGroup.GetMembers() {
 			server, memberAddress, err := GetServerFixedIP(t.Cloud.ComputeClient(), serverID, fi.StringValue(e.InterfaceName))
 			if err != nil {
 				return err
diff --git a/upup/pkg/fi/cloudup/openstacktasks/servergroup.go b/upup/pkg/fi/cloudup/openstacktasks/servergroup.go
index 0acacd2f86..8abb5740ab 100644
--- a/upup/pkg/fi/cloudup/openstacktasks/servergroup.go
+++ b/upup/pkg/fi/cloudup/openstacktasks/servergroup.go
@@ -19,6 +19,7 @@ package openstacktasks
 import (
 	"fmt"
 	"strings"
+	"sync"
 
 	"github.com/gophercloud/gophercloud/openstack/compute/v2/extensions/servergroups"
 	"github.com/gophercloud/gophercloud/openstack/compute/v2/servers"
@@ -33,10 +34,19 @@ type ServerGroup struct {
 	Name        *string
 	ClusterName *string
 	IGName      *string
-	Members     []string
 	Policies    []string
 	MaxSize     *int32
 	Lifecycle   *fi.Lifecycle
+
+	mutex sync.Mutex
+
+	// members caches a list of member instance names.
+	// When we create new instances, we can add them to this list also.
+	members []string
+
+	// gotMemberList records if we have returned the member list to another task.
+	// If we attempt to add a member after doing so, it indicates a missing dependency.
+	gotMemberList bool
 }
 
 var _ fi.CompareWithID = &ServerGroup{}
@@ -71,10 +81,10 @@ func (s *ServerGroup) Find(context *fi.Context) (*ServerGroup, error) {
 				ClusterName: s.ClusterName,
 				IGName:      s.IGName,
 				ID:          fi.String(serverGroup.ID),
-				Members:     serverGroup.Members,
 				Lifecycle:   s.Lifecycle,
 				Policies:    serverGroup.Policies,
 				MaxSize:     fi.Int32(int32(len(serverGroup.Members))),
+				members:     serverGroup.Members,
 			}
 		}
 	}
@@ -88,7 +98,7 @@ func (s *ServerGroup) Find(context *fi.Context) (*ServerGroup, error) {
 	}
 
 	s.ID = actual.ID
-	s.Members = actual.Members
+	s.members = actual.members
 	return actual, nil
 }
 
@@ -157,3 +167,29 @@ func (_ *ServerGroup) RenderOpenstack(t *openstack.OpenstackAPITarget, a, e, cha
 	klog.V(2).Infof("Openstack task ServerGroup::RenderOpenstack did nothing")
 	return nil
 }
+
+// AddNewMember is called when we created an instance in this server group.
+// It adds it to the internal cached list of members.
+// If we have already called GetMembers, this function will panic;
+// this can be avoided by use of GetDependencies() (typically by depending on Instance tasks)
+func (s *ServerGroup) AddNewMember(memberID string) {
+	s.mutex.Lock()
+	defer s.mutex.Unlock()
+
+	if s.gotMemberList {
+		klog.Fatalf("attempt to add member %q after member list already returned", memberID)
+	}
+
+	s.members = append(s.members, memberID)
+}
+
+// GetMembers returns the ids of servers in this group.
+// It also activates the "flag" preventning further calls to AddNewMember,
+// guaranteeing that no new members will be added.
+func (s *ServerGroup) GetMembers() []string {
+	s.mutex.Lock()
+	defer s.mutex.Unlock()
+
+	s.gotMemberList = true
+	return s.members
+}