code: func (c *MemoryGroupService) GroupAddMember(ctx context.Context, groupName, uid string) error {
  	memoryGroupsMu.Lock()
  	defer memoryGroupsMu.Unlock()
  
  	mg, ok := memoryGroups[groupName]
  	if !ok {
  		return constants.ErrGroupNotFound
  	}
  
  	_, contains := elementIndex(mg.Uids, uid)
  	if contains {
  		return constants.ErrMemberAlreadyExists
  	}
  
  	mg.Uids = append(mg.Uids, uid)
  	memoryGroups[groupName] = mg
  	return nil
  }
docs: // GroupAddMember adds UID to group