UPDATE w3t_Users SET U_Groups = '-3-5-' WHERE U_Totalposts > 100 AND U_Status = 'User'
Note: That will change whatever user groups that they had - to -3- (the user group) and -5- (or whatever other group you need).
If you need to append the new user group to what they have.... I'm nor sure exactly what needs to be done. I'd write a little script for it. (Contact me and I'll help you AA).
RandyJG posted here about the use of CONCAT but I'm not familiar with it myself.
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