Hello Faith Webster
Whether you are using ArcGIS Online or Portal for ArcGIS each user should have their own account to sign into Collector. Each user will then download an offline web map on their individual devices. It is possible for multiple users to sync to the same hosted feature service.
If for example two users edit the same attribute in the same record in a hosted feature service. The user who sync last would overwrite the previous user edits. This would mainly be an issue if two users were editing the same record.
If you are using portal and publishing a referenced feature service you have the capability of using versioning. Versioning can allow users to edit their own version of the data. When going through the reconcile and post process if there are conflicting edits you can then determine which edits to keep.
Traditional Versioning:
Esri Support ArcGIS Server 10.8.1
Hope this helps!
Thanks,
Thomas.