Formal:
* A company of travelers
* A party of travelers
* A group of travelers
Informal:
* A band of travelers
* A throng of travelers
* A crowd of travelers
* A gaggle of travelers (implies a somewhat chaotic group)
You could also use a more descriptive term, like:
* A caravan of travelers (suggests a group traveling together)
* A pilgrimage of travelers (implies a religious journey)
* An expedition of travelers (suggests a group on a mission)
Ultimately, the best collective noun to use depends on the specific situation and the tone you want to convey.