a. I'll assign the project to you in three days.
b. I'll assign you the project in three days.
Both sentences are grammatically correct and you can use them. The difference between a and b lies in the position of the DIRECT OBJECT (i.e the project) and INDIRECT OBJECT (i.e you). The direct object receives the action (verb) directly while the indirect object receives the action (verb) indirectly.
a. Subject + Verb + Direct Object + "to" + Indirect Object.
b. Subject + Verb + Indirect Object + Direct Object.
Hope this helps.