One wild Baldur's Gate 3 player has devised a method for keeping Wyll while also helping Minthara with her raid on the Grove.
I love Wyll, just as I love Minthara, but only one of them loves raiding the Grove and slaughtering helpless Tieflings. If you tell Minthara where the Grove is, thus opening her up to join your party later on, you're forced to say goodbye to Wyll, who abandons you in disgust at your actions. Honestly, considering you just got a lot of innocents killed in the name of Minthara, I'm sort of on his side.
But then again, Minthara is quietly one of the best companions in Baldur's Gate 3. This is why players began devising ways of letting Minthara join your party, while also retaining beloved characters like Karlach and Wyll, both of whom would normally abandon you. Now, one player has come up with the tricky method just below for keeping Wyll around, but it does involve killing him.
Found a way to raid the grove and keep Wyll from r/BaldursGate3
Firstly, you need to steal the Idol from the Grove, thereby setting in motion events where the Tieflings get massacred by the wizards, and not Minthara. Then tell Minthara about the Grove, and have a nice Long Rest. Travel to the Grove, where you'll find the Tieflings already dead, and Minthara will have raided the Grove without you.
Unfortunately, you then need to kill Wyll, and speak to Minthara to initiate the big Goblin victory party. Simply revive Wyll after the party, and in theory, he should join your party with open arms. Being killed apparently gave Wyll amnesia and made him forget that you caused the slaughter of the Tieflings and also told Minthara about the location of the Grove.
If this all works, then it's happy days. The method's creator adds that this may very well work with Karlach, but they unfortunately already beheaded the Tiefling warrior in this run. As you do.