Bridal luncheons are a way for the bride to say "thank you" to her loyal attendants who have been by her side, put together a bridal shower, purchased expensive wedding attire, planned a bachelorette party and calmed her down in the most "bridezilla" moments. These get-togethers are not required by general wedding etiquette so there are no rules for them. This means that invitations to bridesmaid luncheons are also free of formal wedding etiquette and rules. They can be sent in whatever form the bride wishes, so long as she remembers to include all of the important details.
Tell your bridesmaids what the invitation is for. A part of your invitation, usually towards the top, indicates that the invite is for a bridal luncheon to thank them for everything they have done for you. Some brides choose quirky little poems to convey this.
Let them know what you have planned for the luncheon. You may even treat your gals to spa day or ballgame instead of a conventional brunch at a nice restaurant. They need to have some idea of what you have in store so they can dress appropriately.
Include the details. This includes the time and place of the event. If you are meeting at a location separate from the event venue, make sure that information is also included.
Ask them to R.S.V.P. Chances are that all of your bridesmaids are attending the luncheon, but it is still a good idea to include a line at the bottom of the invitation with your phone number or email address requesting that they R.S.V.P. This way you can plan accordingly if someone is not going to be able to make it.
Do not be afraid to be creative with your invitations. A bridal luncheon is often a well-deserved break from the formality of other wedding planning and the invitations are no different.