One line of code does the trick:
new <- ddply( df, "group1", transform, numcolwise(mean))
new
group1 group2 values meanValue
1 1 A 0.48742905 -0.121033381
2 1 A -0.04493361 -0.121033381
3 1 C -0.62124058 -0.121033381
4 1 C -0.30538839 -0.121033381
5 2 A 1.51178117 0.004803931
6 2 B 0.73832471 0.004803931
7 2 A -0.01619026 0.004803931
8 2 B -2.21469989 0.004803931
9 3 B 1.12493092 0.758597929
10 3 C 0.38984324 0.758597929
11 3 B 0.57578135 0.758597929
12 3 A 0.94383621 0.758597929
identical(df, new)
[1] TRUE