When building workflows it has been witnessed that sometimes groups will not appear in participants section of the User Task wizard.

Whenever Appit is installed on a site collection the title of that site collection is used to name the GroupProvider in the K2.HostServer.GroupProvider table with the SQL Azure DB. This GroupProvider will be called by name whenever Appit tries to reference a group. If you are not using a SharePoint onPrem instance of Appit and you notice groups not showing up, the first thing you should try is taking the following path:

-Site Settings>Look And Feel>Title,Description and Logo

If there is no title for the site collection, give it one and then run the registration wizard. This should properly resolve the GroupProvider for the site collection within SQL Azure and properly serve up groups.