Write a thank you note to the whole group, don’t send a card. Since they went to the trouble to chip in for a gift, writing short but sincere note is the way to go.
You could also consider using a greeting card software. Purchasing the software is going to be cheaper than having the invitations professionally printed. You can also reuse this software for several other occasions. There are some Microsoft Office product like Microsoft Word and Publisher that would also work great for you to create your own engagement party invitations. They have already made designs and the Microsoft clip art can add visual enhancements to your engagement invites.
Personalize each note. Don’t send the same, generic message to each guest; it sends the message that you were tired of writing notes and too lazy to personally acknowledge them. Take the time to pen something individual and thank them for their specific gift by name. You may even want to mention how you plan to use it.
Handwritten invitations are perfect for someone who has an attractive handwriting. This saves money on the printing and it gives a personal and handmade touch too. The style of your engagement invitation card is entirely up to you. You can go for casual or informal cards, just like how your party is going to be. If you are planning on a formal event, then you must mention in your invite about any special dress considerations that is expected, so that your guests can know.
You can include anything that you wish like dried flowers that will give earthly scents and make it colorful and accessorize your invitation to give a unique touch for your entire visitors.