Adding more onto an esc_url [closed]

To concatenate strings, PHP uses the concatenation operator .. Thus, to append a string to the current user link, you can use

bp_core_get_userlink( get_the_author_meta( 'ID' ), false, true ) . '/posts/'

Note that I’ve removed the first part of the assignment from the $no_anchor and $just_link parameters, as the values should be passed, and not the assignments themselves.

In general, I suggest you start by reading a few basic PHP tutorials.