Sending the public group link to a private group in order to invite members:
To invite the members of a private group to a public group via group link, click on the group name at top of the screen to see Setting options.
Choosing “Update Group Username” can make a new page open and your group “Username” is shown in its field.
You have this authority to copy and send this username beginning with @ as an invitation link to a private group. So, iGap users in a private group can join that mentioned public group by clicking the invitation link simply.