  Centrality measures characterize important nodes in networks. Efficiently
computing such nodes has received a lot of attention. When considering the
generalization of computing central groups of nodes, challenging optimization
problems occur. In this work, we study two such problems, group-harmonic
maximization and group-closeness maximization both from a theoretical and from
an algorithm engineering perspective.
  On the theoretical side, we obtain the following results. For group-harmonic
maximization, unless $P=NP$, there is no polynomial-time algorithm that
achieves an approximation factor better than $1-1/e$ (directed) and $1-1/(4e)$
(undirected), even for unweighted graphs. On the positive side, we show that a
greedy algorithm achieves an approximation factor of $\lambda(1-2/e)$
(directed) and $\lambda(1-1/e)/2$ (undirected), where $\lambda$ is the ratio of
minimal and maximal edge weights. For group-closeness maximization, the
undirected case is $NP$-hard to be approximated to within a factor better than
$1-1/(e+1)$ and a constant approximation factor is achieved by a local-search
algorithm. For the directed case, however, we show that, for any
$\epsilon<1/2$, the problem is $NP$-hard to be approximated within a factor of
$4|V|^{-\epsilon}$.
  From the algorithm engineering perspective, we provide efficient
implementations of the above greedy and local search algorithms. In our
experimental study we show that, on small instances where an optimum solution
can be computed in reasonable time, the quality of both the greedy and the
local search algorithms come very close to the optimum. On larger instances,
our local search algorithms yield results with superior quality compared to
existing greedy and local search solutions, at the cost of additional running
time. We thus advocate local search for scenarios where solution quality is of
highest concern.
