Voltage follower with shift
The classic 4-resistor difference amplifier will do what you want:







simulate this circuit &ndash; Schematic created using CircuitLab

This circuit keeps the difference between Vout and Vref equal to the difference between V+ and V-, effectively adding Vref to V+ if V- is grounded.

The opamp tries to keep its two input terminals at the same potential using negative feedback:

$$ \frac{V_{out} + V_-}{2} = \frac{V_{ref} + V_+}{2}$$

Multiply both sides by 2 and rearrange the terms to get:

$$V_{out} - V_{ref} = V_+ - V_-$$