Lie algebra associated to an arbitrary discrete group I read somewhere that there is a classical (due to Philip Hall?) construction of a Lie algebra associated to any discrete group $\pi$ which is obtained from filtration quotients of the descending central series of $\pi$. Can anyone recommend some introductory material on this construction?

This is in Lazard's thesis (which may even have been mentioned in the linked MO question?)
Sur les groupes nilpotents et les anneaux de Lie:
http://archive.numdam.org/ARCHIVE/ASENS/ASENS_1954_3_71_2/ASENS_1954_3_71_2_101_0/ASENS_1954_3_71_2_101_0.pdf
Specifically Theorem 2.1 and Corollary 6.8. But the whole paper is basically about this, so all sorts of "relevant facts" are included.
Also relevant is Quillen's paper "On the associated graded ring of a group ring" which proves that the associated graded ring of the group algebra of a p-group in characteristic p (with respect to its augmentation ideal) is the same as the universal enveloping algebra of the associated restricted Lie algebra.
