You can set friction on the solver, and also as an attribute on objects. On the Forces tab of the Vellum Solver SOP, there is a Static Threshold parameter which is controlled by the friction attribute, and a Dynamic Scale parameter which is controlled by the dynamicfriction attribute. Both of these are multiplied by the corresponding attribute on the two things that are colliding. For example, if you have two pieces of cloth colliding against each other, it will take the friction of one multiplied by the friction of the other, multiplied the global static friction on the solver, which will become the static friction for the collision. The same applies for the dynamic friction attributes.