With the end of support for Windows XP, lots of groups will find themselves wanting to have "upgrade parties." Since the group has many common goals and uses for their computers, these group upgrades would be a great way to draw more members and come together on a common subject. Many large corporations and special groups have already moved to Linux for their specific work process. To get your group going, it only takes one or two linux users to help get all the others using the same Linux set of tools.
It is important that you understand what the challenges are before you agree to do a group upgrade. Consider that some users will have so little knowledge of how the computer works, that upgrading to an Android table might be a better idea. Remember too that many of these XP systems were upgraded from other versions of windows and might be 10 or more years old. However, Linux can support the older systems, it is knowing how to support them that is the problem.
Since your going to be upgrading many different types of systems, your going to need a collection of CD/DVD disks and even some flashdrives loaded with linux installs, gparted hard disk utilities, and support packages. For the really old systems, you will want "puppy" linux and their 'retro" packages. For newer systems, any of the major distributions will work, although many no longer come on CDRoms, but only DVDs. If your location for doing the upgrades includes WiFi or internet, you will need cables, and switches to hook things up. Keep in mind that many of the WiFi adapters are not supported out of the box on linux and will require a separate download to support them.
You may find that some of the systems are too old or buggy to boot with a regular install disk. As the leader you will need to know how to adjust the boot command line to turn off various features that might be causing the problems. The gparted disk, has a "fail-safe" startup option and if that works, just use the same command line as they show when using that option. You might find that some systems are so buggy, that upgrading is not possible, however there are special releases of kernels that can support almost anything and thus some internet searching might return the correct command line or distro to get things going.
Dual Booting is where you have more than one operating system on the hard drive. Linux has great support for dual booting, and can boot off of the same drive or another drive. For the XP upgrades, most users will be happier if they can keep their XP system running until they learn that Linux can solve all their needs. For most members, you will need to "shrink" the XP partition to make room for the linux install. Make sure that everyone knows this before hand, as they will need to do "check disk" and "defragging" of their hard drive before the session. That notice should also remind users to back up their systems before coming, as accidents do happen and they could loose all their old data. "gparted" is the tool of choice for shrinking the disk, but it is faster ( some defrags can take hours ) and better if all the data has already been moved to one end of the disk.
There are not many problems that come up when installing linux that can't be fixed by finding some package and installing it. However, the one that can come up is downloading drivers for the network connection when the network connection is not working. This Chicken/Egg, catch22, or "can't get there from here" problem is easy to fix when doing "dual boot" installs. Just boot back on the XP partition and save the package file. Then reboot to Linux and install the needed package. For non-dual boot, you will need to find another machine, copy or download the needed package on to a flashdrive or CDRom and install from that on the problem drive. Some old systems might see a similar problem with video cards, however you can generally load what is needed from the command line. As a leader of the upgrade session, you will need to be ready to step in and type some commands for those few cases that refuse to work any other way.
There will be some members who don't bring anything to upgrade, but are very interested in seeing linux in action. For those users, you need to have some live/install disks ready to give out. Since these will be used on older systems, try and have CDRom disks mostly with a few DVDs. It is possible to create 'demo" disks that are specific to your groups interests and can show how some of the programs work. Keep in mind that once you have users on Linux, you might need to have a "what to do next" session to make sure everone comes up to full speed on using Linux.
An example of XFCE i386 440MB image - with install.
List of sections in "wheezy" repositories.
Debian's Live CD WIKI page - great place to start!
Debian Live systems main page - docs and image builder.
Debian Live-build manual page - all you need to know about live images!