This command assigns a taxon to the outgroup. The correct usage is:
outgroup <number>/<taxon name>
For example, "outgroup 3" assigns the third taxon in the matrix to be the outgroup. Similarly, "outgroup Homo_sapiens" assings the taxon "Homo_sapiens" to be the outgroup (assuming that there is a taxon named "Homo_sapiens" in the matrix). Only a single taxon can be assigned to be the outgroup.