I don't have a welcome letter for new members (might have to create one), but I do have a welome letter for new parents to the school (i.e. new Pre-K, Kindergarten parents, new memebers to community), and a letter of invitation for membership to the families of students who have been in the school for at least a year already. I will be glad to share them if you think they will fit your needs.
The basic outline should include your welcome statement, reinforce to the readers the need for family involvement, and briefly explain that there is no real definitive lines to "involvement", include a short list of programs and activities sponsored by your group and a short list of your goals for the year. Always attach two membership forms (or a form with enough space for two names) to encourage both parents to join, and be sure to include names and contact numbers for officers and committee chairpersons.
My PTA has put together a welcome packet for incoming Pre-K and Kindergarten parents and also obtained a list of newly registered students so a packet can be sent home with them as well. It includes the welcome letter, list of officers and chairpersons, a list of PTA, Board of Ed. meetings and other important dates, first newsletter, survey, membership forms, and addressed dues envelope.
We also have a similar packet for the first and second graders in the school, except it is geared towards parents already familiar with the school. It goes on to include more information on our PTA, and lists some of the changes being made from last year.
One thing I have learned is that the letters do not have to sound "business-ey". A lot of parents (and teachers) feel more comfortable with a light-hearted, good-humored, down-to-earth letter, and feel less intimidated knowing that it is written by "one of them".
If you are interested, email me... This email address is being protected from spambots. You need JavaScript enabled to view it. ... I'll be glad to share...I will let you know what I come up with on the new members letter as well...