I tried the following script but didn't get the desired result.
Script ::
import com.atlassian.jira.component.ComponentAccessor
return ComponentAccessor.getGroupManager().getGroupsForUser(issue.getAssignee())
Result ::
[com.atlassian.crowd.embedded.impl.ImmutableGroup@5ab0a9b9, com.atlassian.crowd.embedded.impl.ImmutableGroup@aa6f0adf, com.atlassian.crowd.embedded.impl.ImmutableGroup@1845f18a, com.atlassian.crowd.embedded.impl.ImmutableGroup@86a, com.atlassian.crowd.embedded.impl.ImmutableGroup@7403ff3, com.atlassian.crowd.embedded.impl.ImmutableGroup@e239b761, com.atlassian.crowd.embedded.impl.ImmutableGroup@42b66b45, com.atlassian.crowd.embedded.impl.ImmutableGroup@9099bd68, com.atlassian.crowd.embedded.impl.ImmutableGroup@852dcdc5, com.atlassian.crowd.embedded.impl.ImmutableGroup@976d6f29, com.atlassian.crowd.embedded.impl.ImmutableGroup@95a62d82, com.atlassian.crowd.embedded.impl.ImmutableGroup@ca5d9308, com.atlassian.crowd.embedded.impl.ImmutableGroup@58e3f5fa, com.atlassian.crowd.embedded.impl.ImmutableGroup@2d90ea2a]