Manual:$wgAutopromote

Details
This array contains criteria of automatic promotion. The syntax is: cond here may be: The latter has following syntax: There are 4 operands available: Condition can be written in two forms:
 * A single condition
 * A set of conditions
 * & (AND) — promote if user matches all conditions
 * | (OR) — promote if user matches any condition
 * ^ (XOR) — promote if user matches only one of two conditions
 * ! (NOT) &mdash; promote if user matches no conditions.

Possible conditions (extensions may add more through the AutopromoteCondition hook):
 * APCOND_EDITCOUNT: minimum number of edits necessary
 * APCOND_AGE: minimum number of seconds since registration
 * APCOND_EMAILCONFIRMED: email address has been confirmed
 * APCOND_INGROUPS: list of groups the user must be in
 * APCOND_ISIP: user has a specific IP address
 * APCOND_IPINRANGE: user is in a specific IP range
 * APCOND_AGE_FROM_EDIT: minimum number of seconds since first edit
 * APCOND_BLOCKED: account is blocked (added in v1.16: )

Caveats
Autopromotion doesn't actually add users to a group; MediaWiki will check whether a user meets the conditions for autopromotion whenever it checks the user's rights or effective groups. This means that a user will only appear to be in a group on Special:ListUsers if they were added to it through Special:UserRights. It also means you can't directly remove a user from an autopromotion group through Special:UserRights: $wgRevokePermissions might be useful for you, or if you're not using MediaWiki 1.16+, a workaround is available.