You don't need several self enrolment methods: just one.
Self Enrolment: enrolment key, yes to group enrolment keys (DO NOT give this key out)
Groups: create the group, make sure you have a unique key for each one. Give the key for the appropriate group to the appropriate people.
Then, when it asks them to self enrol, they enter the group key. It enrols them and puts them in the right group at the same time.