diff --git a/src/group.cpp b/src/group.cpp
index a586c33ed9..b438e8fede 100644
--- a/src/group.cpp
+++ b/src/group.cpp
@@ -623,6 +623,21 @@ int Group::find_unused()
   return -1;
 }
 
+/* ----------------------------------------------------------------------
+   return group bitmask for given group id. Error out if group is not found.
+------------------------------------------------------------------------- */
+
+int Group::get_bitmask_by_id(const std::string &file, int line, const std::string &name,
+                             const std::string &caller)
+{
+  int igroup = 0;
+  while (igroup < MAX_GROUP)
+    if (names[igroup] && (name == names[igroup])) break;
+  if (igroup == MAX_GROUP)
+    error->all(file, line, "Group ID {} requested by {} does not exist", name, caller);
+  return bitmask[igroup];
+}
+
 /* ----------------------------------------------------------------------
    add atoms to group that are in same molecules as atoms already in group
    do not include molID = 0
diff --git a/src/group.h b/src/group.h
index 0820c7d4b2..728b0a1d02 100644
--- a/src/group.h
+++ b/src/group.h
@@ -31,11 +31,13 @@ class Group : protected Pointers {
 
   Group(class LAMMPS *);
   ~Group() override;
+
   void assign(int, char **);                  // assign atoms to a group
   void assign(const std::string &);           // convenience function
   void create(const std::string &, int *);    // add flagged atoms to a group
   int find(const std::string &);              // lookup name in list of groups
   int find_or_create(const char *);           // lookup name or create new group
+  int get_bitmask_by_id(const std::string &, int, const std::string &, const std::string &);
   void write_restart(FILE *);
   void read_restart(FILE *);