A group letter is just the same as a standard letter accept there are multiple senders. You should take the time to write the heading, greeting and body of the letter as you would any other letter. However, there are just a few slight changes you might need to make to the paragraphs of the letter that you would not need to if there was only one sender. You will also have to make specific changes to the ending of the group letter.
Head the letter with the name of the group. If there are multiple senders, you can use the name of the group, such as the business name, department name or name of the organisation, in the header, rather than just one person's name.
Use pronouns that fit with the group letter in the body paragraphs. For example, instead of saying "I feel a change needs to be made" say "We feel a change needs to be made."
Leave plenty of space at the bottom of the group letter for all members of the group to provide their signature. Type a line where each signature will go to make it easier.
Type each sender's name below the signature lines to signify where each person will sign and to make it clear who the sender's of the group letter are.