I use ADMIXTURE frequently but have not done a dendrogram. I pulled up some test data for K=3 and did this in R on the Q values.

admix <- read.table('test.3.Q',header=TRUE,row.names=1)
d <- dist(admix)
h <- hclust(d)
dend <- as.dendrogram(h)
plot(dend)

This is quick and dirty, you can play with the dist, hclust, as.dendrogram and plot functions.

If you are interested I have a [R] script to automatically make ggplot barplots for the admixture across different K values.

Hi Tonig:

If you want to create a phylogenetic tree using your GWAS data I would highly recommend you to use a standard genetic distance for the distance matrix and not just an euclidian distance. For example, you could calculate Fst values (between your populations or individuals) and then use this matrix to create your tree using PHYLIP or any other program (MEGA is also very easy to use).

Fst values to compute NJ trees when using GWAS data is what most authors do.

Best,

Javier