Using the option "enrol users" in "participants" should allow to specify..

• groups the user should belong to
• a specific start date (which might be in the future - at the moment the option is there, but it's limited to the official course start date (which is irrelevant) and "today".)
• a specific end date (so I don't have to calculate the number of days)

Currently I have to

1. enrol the user
2. use the option "edit enrolment" in the participants list to specify a start and end date (it works fine here, why not during enrolment)
3. add the user to groups as necessary

It's fine to do 2) and 3) if I have to edit an enrolment (days or weeks) after initially creating it, but I don't see why it shouldn't be possible to set all those things at the time of enrolling a user.