Hi, 

I cannot invite users to existing groups.

- The user already exists in a different group.

- I have tried to invite them from within the Contact module.

- From within the All Contacts category, I used the "Add To" menu to add 'the selected contact' to the desired group.

- I got the yellow box drop down message saying, "contact successfully added to group"

- The user did not receive an invitation and the spam has also been diligently checked.

- This should be very simple to do. It worked one time and not the next. If there are specific things that must happen in order for these invitations to work, there should be some more information and it should be easily found.