Even though Team Yell is officially the Antagonist Group of The Galar Region, they really aren't all that bad. Much like Team Skull of the 7th Generation, Team Yell simply doesn't compare in terms of level of evil as other Antagonist Groups of Pokemon Past. Throughout the majority of the Main Story, Team Yell's sole purpose is to cause enough havoc so that Marnie becomes the next Galar Region Champion (even if it is against her wishes). This is because all of them are from the town of Spikemuth, and they have become her biggest fans.
With this goal in mind, Team Yell will mainly be small roadblocks you must go through on your way to becoming the next Galar Region Champion. In each Encounter with them, all you will have to do is defeat them in battle to stop their minor misdeeds. After making it to and through Spikemuth on your way to facing Piers, the Spikemuth Gym Leader, Marnie will halt all the mischief of Team Yell once and for all.
From there on out, Team Yell will be no problem for you (though not much of one in the first place). Team Yell will also come to the aid of your trainer while in Wyndon, helping you and Hop make it through the wave of Macro Cosmos Members in order to reach Leon and Rose at the top of Rose Tower. From there, Team Yell can only be found in their hometown of Spikemuth.