Cast in thy lot - Be a frater conjuratus, a sworn brother, and thou shalt have an equal share of all the spoil.
Common sense must teach us that the words here used are such as must be spoken when a gang of cutthroats, pickpockets, etc., are associated together.

Cast in thy lot among us; let us all have one (m) purse:
(m) He shows how the wicked are allured to join together, because they have everyone part of the spoil of the innocent.

Cast in thy lot among us,.... Or "thou shall cause thy lot to fall among us" (u); though just entered, as soon as any booty is taken thou shalt cast lots with us, and have thy full share with those that have been longer engaged;
let us all have one purse; or "we will all have one purse" (w); will throw all our booty, taken by us into one common stock, and live upon it comfortably and merrily. Jarchi represents it as putting it to the young man's option, to do which he would, either to cast lots and take his share separately, or let it be put altogether, and so partake jointly with the rest. According to Gersom the sense is, that there should be such an exact division made, that there should not be more in one purse than in another; their shares should be equally divided by lot, and their purses should be alike; one should not have more than another: these are the arguments used by wicked men to allure and ensnare young men to join with them in their sinful ways and practices; from which they are dehorted, as follows.
(u) "sortem tuam conjicies", Junius & Tremellius; "projicies", Mercerus, Baynus; "jacies", Cocceius, Michaelis, Schultens. (w) "erit nobis omnibus", Pagninus, Montanus, Tigurine version; so Cocceius, Schultens, and the Targum.