Yes, it is possible to update entire user groups fairly quickly if you have the Data Import/Export App enabled in your site.
First, make sure the user groups in question have an Import/Export Key defined. This can be the name of the user group with no spaces. For detailed instructions on creating Import/Export Keys, please refer to the Data Import/Export App Knowledgebase article.
After defining Import/Export Keys for each user group, use the Import/Export app to export all users belonging to the desired user group (or all if you need to make changes to all your users).
Make any desired changes to the user group columns for each user in the CSV file, save it and re-import it back into VolunteerHub. For example, to make a user a member of a user group, you would change their membership column response from FALSE to TRUE.
Be very careful not to accidentally change other user information when doing this. Once you import the file back into VolunteerHub, your changes will not be reversible!