Personal Interest Grouping is where your app users will select the groups in which they want to be a part of (rather than you, the app administrator, assigning them their groups).
On the login screen for the app users will select from a menu any group(s) they want to be a part of. After selecting their groups on the login screen, the menu of groups can also be accessed via the Settings icon so they can always change the selection at a later date.
To setup the personal interest grouping you will provide a list of the groups to your Account Manager. From there your account manager will get them into the app.
To allow for the best user experience we recommend no more than 15 different personal interest groups but ultimately it is up to the event organizer/app admin.