Stay home.
As demonstrated in the previous comments, you can't know the health status of the other participants. Yes, it is true that immuno-compromised people take calculated risks of exposure to live a normal life, and we are all exposed to who knows what when we leave our homes (whether due to people who are sick pretending they're not, or because they simply don't have symptoms yet), but that is different than a social activity, where there is (presumably) more personal interaction, shared tools, etc.
So, OP, I guess someone should ask: what sort of activity is it?