NO NEED TO ADD OR FIND GROUP UUID's
Just copy the script add it to a prim and set the prim to the group you want people to join.
When touched a message will appear and in chat history it will show a link to the group, clicking this link will automatically open the group. Note: Group must be set to Open enrollment.
Does not work on some older 1.18 client versions as the blue link in chat history fails to display as a link.
I will drop a copy in SLexchange in the next 24 hours as well for zero lindens.
******************************************
//This script is not for resale but is available to distribute freely.
default
{
touch_start(integer t)
{
string groupID =llList2String(llGetObjectDetails(llGetKey(), [OBJECT_GROUP]), 0);
integer i;
for(i = 0; i < t; i++)
{
llInstantMessage(llDetectedKey(i), "add your message here for you new group members, Please Click this link! secondlife:///app/group/" + groupID + "/about"

}
}
}
******************************************
This script if my memory serves me rightly may be in the Library already but I could not find it in a search prior to posting.