Sarah, along with her colleagues, __________ organizing the charity event.

Complete Sentence: Sarah, along with her colleagues, is organizing the charity event.

• along with (idiom)
 English Meaning: together with (something or someone).
Bangla Meaning: সঙ্গে যাওয়া।

→ with, together with, as well as, along with, in addition to, accompanied by দ্বারা কোন Noun বা Pronoun যুক্ত হলে verb টি প্রথম Subject অনুযায়ী হয়। 
→ Sarah এবং her colleagues, along with দ্বারা যুক্ত হওয়ায় Sarah অনুযায়ী verb টি singular হবে অর্থাৎ, is হবে। 

Source: Advanced Learner's English Grammar & Composition by Chowdhury & Hossain.