General Antonyms:
* Stand tall
* Be dignified
* Maintain one's pride
* Be assertive
* Assert oneself
* Demand respect
More Specific Antonyms:
* Dominate (if groveling implies submission)
* Confront (if groveling implies avoidance)
* Reject (if groveling implies acceptance)
* Refuse (if groveling implies compliance)
* Challenge (if groveling implies deference)
The best antonym for "grovel" will depend on the specific situation and the intended meaning.