I am organizing a weekly seminar (at specific day of every week) at our department. We will have 12 talks for the next coming semester. We have a list of n potential people for these talks (suggested by seminar committee). It is clear that not every people on this list can actually deliver a talk (for time restrain or personal reasons), so n should be larger than 12.
The invitation is done by sending an email to the potential persons, asking if he/she is interested to give a talk. We don't necessarily have a phone number of these people.
What is the best approach to propose the date of seminars to the interested people by email?
If I list all the available 12 dates to the first person who replies positively, he/she may not quickly select the desired date and asking to answer quickly may be sometimes uncomfortable. If I have his/her phone number I immediately call him/her for fixing the date, but some people don't like to share their personal phone numbers, also office public phone numbers are not always working (once I almost spent the whole working day to call someone at his office but nobody was there). Also while I am waiting for someone to select his date, it is difficult to propose the same dates to another person as both may select the same date.