BuddyPress.org

Which version of BuddyPress are you running? The current version of BP does not have that function. Instead, it is bp_has_members().

Having you been developing under an older version and then you upgraded? Codebase changes are to be expected. The first thing to do when previously working code fails is to check the logs, they will often indicate the issue. Then, if you need to sleuth some more, check the core functions to see if something has changed–the function name, function parameters, or both.