Add persons to group by username

We need a view that allows a two-step process:

1. Select a group
2. Enter single usernames, in a loop (ask for username, and on submit, load the username step again)

This will be used for workshop and activity subgroups. We will create a group for "ESP-Rennauto", then scan the barcodes of all the participants with a barcode scanner to record them into the group.