Green’s Dictionary of Slang

round up v.

[Western imagery]

(orig. US) to assemble, to get together.

[US]D. Runyon ‘Social Error’ in Runyon on Broadway (1954) 457: I am just wondering [...] if I cannot round myself up with her.
[UK]G. Kersh Fowlers End (2001) 97: Mr. Laverock wants to give the staff the once-over. So round the scum up.