You can use the calendar Map view to assign multiple visits at once to a user..
Assign a single visit
If a visit is not assigned, it will show the red 'unassigned' icon in the team section of the visit details. To assign a single visit, click on it, then click 'Assign'.
A box will come up, where you can can click ' Assign' and select your team. You can then update the visit.
Assign multiple visits
To assign multiple visits to a single user, click on their name at the top of the map view. The other users will turn white and the user you've selected will be highlighted. You can then click on the visits in either the task list on the left, or the map, to assign them to this user.
Once you are finished selecting visits, click Save at the top right to assign them all.
Once you save the visit selection, the map view will show the route for the user. You can use Daily Route Optimization to optimize the route for each user. Routing on the map only works for unscheduled visits. If a visit has a set time, it will not be part of the route.