You can invite any Palette subscribers to your group. If you are creating a group for the first time, you can invite members by typing in their emails. Be sure to use a new line for each email. They will have to accept your invitation to join the group.
If you are inviting new members to an existing group (you have to be an admin of the group), click on 'Groups', click on the 3 dots under the group, and click 'Invite'
You can invite any Palette subscribers by entering their names. They will have to accept the invitation to join the group.