while(<DATA>){
	print "\n$1:" if s/^(\d+ \w+)//;
	print " $1" while(s/(\w+\d+)//);
}


__DATA__
1 change by user23 
8 changes by 4 users: user1 user2 user3 user4
6 changes by 4 users: user5 user3 user3 user1 user15 
1 change by user1