When you create Groups in SCOM you have the option of populating those groups with Explicit members or Dynamic members. If you don’t know what the difference is, basically “Explicit” Group Membership is where you list those agents that should be part of the group. In a Dynamic Group you can use Regular Expressions and Wildcard matches to add Group Members. But there is one other very important difference.
When you delete an Agent from a Server then that Agent is removed from all of the Explicit Groups that the Agent is a member of. If you add that agent back in, the agent will not be a member of the groups it was previously a member of.
In contrast if an Agent was a member of a Dynamic Group then it will automatically be added back into the group it was previously a member of. For that reason I prefer to use Dynamic Groups wherever possible.