I'm going to explain what things are going to make life difficult for the new Testers on your team and what steps you can take to avoid them.
It can be very disorienting for someone that hasn't worked this way before to find their feet, particularly for Testers. When you get this right, you end up with an energised and productive new colleague. A colleague that makes the skillset of the team more T-shaped.
#1: Walk them through the tech stack as soon as possible and make yourself available for follow-up questions.
The easier it is to onboard a new team member, the better.
What would you prefer?
Teammates unable to perceive risk because they lack clarity on how the app works?
Or teammates with a good grasp of the architecture that thoughtfully considers risks and handles them appropriately?
It's not uncommon for a new starter to drown at the firehose of information when first start. Show empathy and expect them to come back with questions.
#2: Remember that Done = Built + Tested.
Working in mini silos defeats the point of agile practices. There is no sense racing ahead because the coding work is "done" but the feature remains untested.
That increases two things.
#3: Whenever you're discussing how to build something, invite the Tester.
There is nothing worse than being isolated or feeling excluded by the rest of the team. It's compounded if the people with the information you need are doing the excluding.
Instead, develop a habit of always inviting your testing teammate when discussing how to implement a feature, especially if it's their first agile rodeo. Allow them to be part of the discussion and manage their own diary.
There are many excellent & sensible reasons not to invite allThePeople to allTheMeetings. Sadly ruling people out of discussions that will be "too technical for them" has an ostracising effect.
Instead, let folks decide for themselves.
An integrated Tester is a productive Tester.
An agile team can be a daunting place for a Tester new to that way of working. Do your best to help them feel valued, and they will repay that in spades!