Contact Names and Indexing--Don't use a comma
I just want to add that I'm having the same issue, this has been an issue for years.
Like Legacy user explains, it's impossible to add someone to a group or sub-group if they have a comma in their name. And commas seem to be the default.
The UI logic will attempt to resolve a name after the first comma, so if someone's name is "LAST, FIRST MIDDLE", when you type the comma between LAST and FIRST, the UI attempts to resolve the 'Contact Name' "LAST".
I've tried to type out the entire 'Contact Name', including the comma in notepad and pasting it into the field.. the UI attempts to resolve the name, but fails again.
These are the workarounds I've found:
1) Copy the URL associated with the group and send it to those you want to invite.
2) Request that people go into their settings, and modify their contact name to NOT have a comma in it. ( I haven't tested this, but hypothetically it should work)