You can use one of the integrated services to order the translations from a third party. Just go to your order settings - order services and enable the service you like to use by providing the needed data.
Using an integrated ordering service might be a good option to start with.
You decided you like to have more control and a direct contact with your translators or translation agency. The benefits of doing so are clear you get more control over your deadline and the quality. But this also means more work for you.
One option to get the work done is exporting the translations using the UI or API and send those to the translators. This is a traditional way - but translators are used to it. But be aware sending files around creates always an overhead.
Invite them to your project
You can invite individuals to your project and limiting access to certain versions and languages.
This option is the best case - but might be limited to your own in-house translators or people you already work with for a longer time.
Check also this advice on how to best keeping track of new translations.
Use the crowdbased service (branched projects)
You like to provide your translators with the benefits of using the locize UI or like to avoid sending files around?
With the service "locize crowdbased" you can create a new branched project on locize containing the untranslated strings per language and share them with the translators.
No need to grant access to your main project
The created projects are connected with your main project - import the translations back with one click
Invite translators to those projects sharing one invitation link (no need for individual invitations per email)