Input a lot of data with select vs checkbox

I want to input some sets of data. Let's say it's like an assignment for a member on a team in some task. I want to apply several members in the task but how is the best experience to input these members.

I have 2 wireframes on the input procedure, the first one is using a searchable select option so I can only input 1 member at a time then press the 'add' button after another.

And the other is a checkbox which I can check the members from a list of the team members so I just need to press the 'add' button once.

Here's the flow I drawn for the assigned procedure.

'add' flow wireframe

TL;DR - If a team can have 40 members, but some task usually has 6-10 members assigned. Which method is the best for assigning the members?

I don't know what tags should I put, help me out...

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