Priority Mapping: Scheduling a training session in between holiday seasons, is not a great idea from employee’s perspective. This not only deter your audience this time but also might prevent them in future as well.
Sticky Subject Lines: Well-chosen words will call for action and shall induce tickle to their neural maps! A short, succinct and relatable tag-line is all what you need. E.g. Training on Email Etiquette may be tagged as “Power Sessions on Powerful Mailings”
What's In It For Me (WIIFM): Their brain (Very much like ours), do not like confusion. Famous saying- People reverts better to those who knows what they want. Therefore mention the training outcome in the invite itself.
Visual Aids: Make your invites more visual and interactive. This will tickle their neurons for much longer duration and compel them to ask for more.
The 'Human' Touch: As per Neuroscience, Human Relations weighs more than any transactional relations. Giving a buzz after the invite might attracts more participants.