We currently have the option to import users into "My Smartsheet Contacts" using a .CSV/.VCF file and this works well in my usage. We don't have the same feature to import a large number of contacts in to a group and I think this would be a good option.
I needed to add >900 users into a group and checked with Smartsheet support to identify the best way to achieve this. Copy and pasting the email addresses into the group was recommended. However, I found that I got an error if I tried to do this with more than 100 contacts in one go and so had to repeat the copy and paste 10 times.