Does Rek'Sai give units Accelerate free or do you have to pay for them?

No, Rek'Sai, Breacher does not make the Accelerate cost free; you must still pay for it if you want the unit to enter the board ready.

According to RiftJudge FAQ #9737 and Rule 805.1.a, Accelerate is an optional additional cost of [1] Energy and 1 Power. Rek'Sai, Breacher’s ability grants the keyword to your units played from outside your hand, which gives you the option to pay that cost.

Here is how the interaction works:

• Optional Cost: You are not required to pay the cost. If you choose not to, the unit will enter the battlefield exhausted (the default state).
• Payment Timing: If you want the unit to enter the board ready, you must pay the [1] Energy and 1 Power during the "Pay Costs" step of playing that unit.
• Tokens: This applies to tokens as well. As noted in RiftJudge FAQ #9360, when an ability plays a token, Rek'Sai, Breacher grants it Accelerate, and you may pay the cost as part of the process of playing that token to have it enter ready.