When creating a Dynamic Group, there are several options available for you to build your group membership list.
The most powerful and customizable option available is "Include by Query" which allows you to build a custom LDAP query, the same way you would to query any standard LDAP service.
If you are looking to return User objects who's Custom Attribute 11 is equal to 100 and who is a member of a certain group, you will have to create a custom search based on an LDAP query.
These queries can be constructed using LDAP syntax and you should refer to any third-party LDAP documentation.
Example of a few queries
The following query will return User objects with Custom Attribute 11 equal to '100'
The following query will return User objects who's a member of 'Group-001'
Combine the above to return User objects that meet both criteria