Well just for an idea why dont you start a piece of collective artwork. Something with four or five canvases. Each canvas can be a side of your alter.
On each canvas can be items that best relate to your magic, things that are personal, things that are spiritual to you, things you have heard from spirits etc.
When all these sides are put together you have your group alter.
Maybe if not all of you can work together the alter changes shape has only three sides etc. Get the idea?
Everytime you use it as an alter add something that represnts the work you did to your canvas.
You can glue things to it, stitch things into it, paint images on it, pour your energy into it.
The other thing is if you call it a piece of group art then you can take it with you where you want, put it where you want, work on it where you want.
Real alters are made of magic. The material part is just a way of focussing your magical attention.